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Topic: change differential ratio

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change differential ratio (1/46)
 8/19/12 3:17pm
dapperdan
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COLUMBUS, NC - USA

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2011 Grand Sport Coupe, 6-speed 1977 Coupe L82, 4-speed


Joined: 8/19/2012
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My '77, L82, 4-speed has a 3.70 rear end.  Chevy did this to give the car some added response.  However, this means that the car is a real gas hog.  At 60 MPH the tach is at about 3500 RPMs.  Am thinking about getting the rear end changed to a 3.53 to get the revs down and thus save on fuel.  It's a big job and won't be cheap, but the car should handle OK.  The only downside is that I will be changing the car, which is totally original with only 30000 true miles. Any thoughts on this?



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Re: change differential ratio (2/46)
 8/19/12 3:49pm
DStoffel
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I changed my 1978 L-82 automatic from the factory 355:1 to 308:1 gears a couple of years ago for the same reason. I definitly lost some kick out of the hole but gained some top end.
A tranny with 1 more gear would have been a better solution but too expensive. It cost me about $500 to have diff. rebuilt plus the cost of the gears. I did the removal & reinstall myself.
I saved the original gears just in case I or a future owner wanted to switch back. You would also want to change the driven gear in the tranny so that your speedometer will be correct. I'm not sure you would see a huge difference with the gear change you are talking about.
 
Dave


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Re: change differential ratio (3/46)
 8/19/12 7:58pm
corvette440hpLifetime Member
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DStoffel said: I changed my 1978 L-82 automatic from the factory 355:1 to 308:1 gears a couple of years ago for the same reason. I definitly lost some kick out of the hole but gained some top end.
A tranny with 1 more gear would have been a better solution but too expensive. It cost me about $500 to have diff. rebuilt plus the cost of the gears. I did the removal & reinstall myself.
I saved the original gears just in case I or a future owner wanted to switch back. You would also want to change the driven gear in the tranny so that your speedometer will be correct. I'm not sure you would see a huge difference with the gear change you are talking about.
 
Dave

Dave is right about the minor change that you would see. If you do change you need to go with the 308:1. But the overdrive transmission is really a better approach.
ps: I like your cat.......take care,
Larry

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Re: change differential ratio (4/46)
 8/20/12 1:21am
rod7515Lifetime Member
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Red Lion, PA - USA

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Dan
Welcome aboard! I went the other way for gearing but I will be driving mine very little. With the 4 speed trans Im not sure what pulling out would be like if you went to 3:08 gears ecspecially if you are on a steep hill.   The others are correct when they say going to 3:55 gears may not yield much of a change  Is this your daily drive? Also as said in the other posts you will need to change the gear in the trans if you want the speedo to be correct. Going from 3:73 to 3:55 you might not need to worry as its probably only a few MPH difference. Keep us filled in as to what you decide and best of luck in which ever direction you go.
Rodney


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Re: change differential ratio (5/46)
 8/20/12 1:53am
daveo76
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Vancouver, WA - USA

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Another option might be 3.36 which was standard for an L-48 4 spd in the mid 70's.  It's what mine has and I feel like it has the right combo of good low end without being too much of a screamer at 70 mph.  (I think it's around 3K at 70 or thereabouts)

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Re: change differential ratio (6/46)
 8/20/12 8:09am
DaveMLifetime Member
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Flanders, NJ - USA

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Going from 3.70 to 3.53 is not much of a diff. It would result in going from 3500 rpm to 3358 rpm. I don't think the sayings on fuel would justify the cost of changing the rear.
I have 3.55 gears and a R4-700 OD trans and at 70 mph I'm running at 2200 rpm. And that trans has a wicked 1st. gear so off the line it still pops.
Just more food for thought.
Dave


|UPDATED|8/20/2012 5:09:50 AM (AZT)|/UPDATED|


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Re: change differential ratio (7/46)
 8/20/12 11:39am
dapperdan
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COLUMBUS, NC - USA

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Thanks for the feedback to all.  Sounds like the 5 speed tranny is the best option, but I know that's not a cheap conversion.  I agree that the slight change to a 3.55 would not gain me much.  I like the compromise--3.36.  Also, there were two different transmissions, I believe, a 2.64:1 and a 2.43:1.  I must have the 2.64.  I think that that means that the first gear is 2.64:1?

I just overhauled the rochester and improved the mpg a little.  I've got the APT adjusted to 2 turns out and it still runs OK.  It was at 3 1/2.  I didn't change any of the jets or rods.

I'm gonna do some research on the differential and check with the guy who has done some of the work on the car.  It needed a new radiator and harmonic balancer and the front oil seal was leaking.  All in all it is in pretty good mechanical shape.


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Re: change differential ratio (8/46)
 8/20/12 1:40pm
Adams' AppleLifetime Member
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Imma gunna throw in with the 3.36 guys too....good all-around ratio. Not too steep, not too lazy. Your L-82 should have the close ratio trans with the higher(lower numerically) 1st gear ratio. It could have been ordered either way. The L-48 engine had the "better" 1st gear ratio transmissions(wide ratio), believe it or not.

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Re: change differential ratio (9/46)
 8/20/12 3:03pm
dskopp
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Oak Creek, WI - USA

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Welcome to the gang, Dan!  Anybody with the name "dapperdan" is ok in my book cause that was my nick name in High School (Many moons ago!!).  I have no expertise on this topic but just wanted to say "Howdy" to you!!
Dan


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Re: change differential ratio (10/46)
 8/20/12 11:11pm
F4GaryGold Member
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I went from a 4.11 to a 3.08 in my LT-1.  Huge change in rpms and mpg.  In retrospect, I wish I had put in a 3.36.  The 3.08
kinda bogs a little with my LT-1.  My 78 L82 had a 3.08 but I think it had a little more low end torque that the LT-1.


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Re: change differential ratio (11/46)
 8/21/12 2:24am
daveo76
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Vancouver, WA - USA

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Yep, when I had my rear end rebuilt 7 or 8 years ago the guy at first had trouble locating a 3.36 ring and pinion set.  I balked a bit and I'm glad I did because I still think it's the best overall ratio for a cruiser that still has its 1:1 top gear original tranny.   The guy ended up finding a 3.36 and I got what I wanted and didn't have to change my speedo drive gear thingy.  

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Re: change differential ratio (12/46)
 8/21/12 8:15am
manchestersharkLifetime Member
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Equinunk, PA - USA

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 The 3:36 works good for me. Gas is around 18, but hole shot suffers. I am not interested in smokin tires anyway. If your gonna drive any distance the TREMEC 5 cog will take "MANY" hours in the white line lane to have it pay for itself in gas savings. If your gonna spin around town and play with compeditors then go with the 5 speed. I was discouraged from going with a 3:08, and it was a good decision.

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Re: change differential ratio (13/46)
 8/21/12 2:32pm
dapperdan
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COLUMBUS, NC - USA

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2011 Grand Sport Coupe, 6-speed 1977 Coupe L82, 4-speed


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How tough was it to do the removal and install of the rear end?

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Re: change differential ratio (14/46)
 8/21/12 2:35pm
dapperdan
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COLUMBUS, NC - USA

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Several others have recommended the 3.36 over the 3.08.   I am checking that out now.  The cat in the picture is no longer with us.  He was a great cat! 

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Re: change differential ratio (15/46)
 8/21/12 2:38pm
dapperdan
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COLUMBUS, NC - USA

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Thanks for the advice on the 3.36.  I am checking that out with my tech now.  I don't think that I want to try to get the rear end out on my own.  My tech has done a lot of rear end repairs.  He had one on his bench yesterday when I talked to him.

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Re: change differential ratio (16/46)
 8/21/12 2:39pm
dapperdan
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COLUMBUS, NC - USA

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OK, Thanks for the hello!

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Re: change differential ratio (17/46)
 8/21/12 2:41pm
dapperdan
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COLUMBUS, NC - USA

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That LT-1 must have really revved with the 4.11.  Can't imagine that being good on the open road, but damn good for dragging.

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Re: change differential ratio (18/46)
 8/21/12 2:43pm
dapperdan
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COLUMBUS, NC - USA

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Sounds good to me.  I'm getting some price quotes on the 3.36.  Don't know about the gear in the trans for the speed-o.  Will check that out.

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Re: change differential ratio (19/46)
 8/21/12 2:44pm
dapperdan
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COLUMBUS, NC - USA

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I will proly go with the 3.36.  Thanks for the advice.

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Re: change differential ratio (20/46)
 8/21/12 2:52pm
JohnRR
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Your 3.64 1st. gear times the new 3.36 rear gear ratio yields a nice total ratio of 12.23.
On the other hand, an old Muncie close ratio with a 2.20 1st gear times an original 3.73 only netted a crappy total 8.21 ratio.  I don't believe going to a 3.56 ratio is enough change to be worth the money or effort.

So I like the 3.36 ratio.  Your rpm's and torque will drop by 10% and your MPG should increase a little.

John


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Re: change differential ratio (21/46)
 8/21/12 2:58pm
corvette440hpLifetime Member
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dapperdan said: How tough was it to do the removal and install of the rear end?

Dan that's a loaded question. It all depends on how mechanical you are. Also if you have the tools. You really need to get a sevice manual.................. On a different note: Im sure that you miss your cat......he or she was beautiful.

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Re: change differential ratio (22/46)
 8/21/12 3:54pm
my7t1Lifetime Member
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Dorr, MI - USA

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71 Colonnade Hardtop Coupe Torch Red with black interior originally L48 built to push around 360hp


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Welcome Dan. My 71 L48 has 3:36 gears, 4spd trans. At 70 mph I'm right around 2900 R's. Now the good news besides the mileage I get, my punched out 383, mild cam, and some other toys thrown in I have absolutely no problems breaking em loose and roasting the back tires.
I agree with others that's my ratio of choice.


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Re: change differential ratio (23/46)
 11/15/12 10:47am
tonytheroofer
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Rochester, NY - USA

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1980 L- 82 fully restored and upgraded.


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I would leave the rear alone , the gains in MPG will be little at best and the drive-ability around town will off set any highway gains in MPG you get . So leave the rear alone and swap the transmission because you then maintain all the power and driveablity BUT get positive and solid MPG gains you will notice at the pump.

I having an automatic just dropped in the 700R4 trans with a 3.55 rear it is wow .. You can hardly hear the motor on the expressway in OD. 

Is the trans swap more ? yes however driving experience alone should make you feel like the money was very well spent . 


|UPDATED|11/15/2012 7:47:04 AM (AZT)|/UPDATED|


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Re: change differential ratio (24/46)
 11/15/12 4:01pm
dapperdan
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COLUMBUS, NC - USA

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My '77 coupe is rare.  It is one of 2060 out of about 50,000 that had the combination of an L82 engine and a 4-speed transmission.  I don't want to change the trans.

I had a '76 L82 with a 4-speed many years ago and it had the 3.36 differential.  It was not a screamer, like this '77 model.

The gas mileage is one thing, but the wear and tear on the engine is what concerns me.  My '77 only has 31,000 original miles on it but the engine has been revving way too high throughout those miles.  I want to calm it down some.

I have 2011 GS that I can drive for performance purposes, so I don't care about the '77 as a performance car.  i just like its looks and the way it handles.  It is not going to be driven hard by me.


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Re: change differential ratio (25/46)
 11/15/12 6:00pm
tonytheroofer
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dapperdan said: My '77 coupe is rare.  It is one of 2060 out of about 50,000 that had the combination of an L82 engine and a 4-speed transmission.  I don't want to change the trans.

I had a '76 L82 with a 4-speed many years ago and it had the 3.36 differential.  It was not a screamer, like this '77 model.

The gas mileage is one thing, but the wear and tear on the engine is what concerns me.  My '77 only has 31,000 original miles on it but the engine has been revving way too high throughout those miles.  I want to calm it down some.

I have 2011 GS that I can drive for performance purposes, so I don't care about the '77 as a performance car.  i just like its looks and the way it handles.  It is not going to be driven hard by me.


Well glad you cleared that up after several ideas of it having been mentioned to you.

As far as gears at 60mph you are not hitting 3500rpm ,  unless you installed short tiny tires but that cant be because it is an original 77 right ? Or you have a 4.56 rear gear set other wise ...... At 60mph your car is going to turn 2723 rpm .. Using a 225/70/15 tire .. If it has an optional 255/60/15 it will turn 2758 at 60mph . .If you gear change it you will save 250 rpm with a 3.36 .Nothing to write home about and nothing you will recover expense wise making the change and nothing that will extend engine life. 

PS) you would have to run 75mph to hit 3400 .. if your tach is showing 3500 at 60mph you have an issue because you in no way shape or form could hit that rpm at 60 in a stock 1977 corvette at 60mph



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Re: change differential ratio (26/46)
 11/16/12 12:02pm
dapperdan
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COLUMBUS, NC - USA

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I put a mark on the drive shaft and had my wife turn one of the rear wheels one turn.  The drive shaft turned 3 & 3/4 turns, approximately.  I agree with all of your numbers on the RPMs at different speeds.  I know the formula for computing the RPMs.  I have a new set of Radial T/As on the car:  225/R70/15.  The tire measures about 27 inches in diameter.  Thus, RPMs at 65 mph are calculated:  (65 X 336 X 3.70) / 27 = 2993.  Maybe, I indicated a higher RPM in a previous note, and maybe my Tach and/or speedometer are not quite right.  I was just trying to indicate that the 3.70 rear end is a really low gear ratio for this car.  

I realize that a 3.36 rear end would get me about a 10% reduction in RPMs at all speeds 336/370 = 0.908.  But that would help reduce the general feel and vibration level of the car at highway speeds.  The 350 V8 does not have to run that fast to accommodate the weight and drag of the car.  Chevy put the 3.70 in the performance car to give the owner the feel that the car was responsive, I think???The same car in 1976 had a 3.36 as the standard differential.  I don't think the '76 had an A.I.R. pump, so it had a little more rear wheel HP.

I got a quote from Keisler Engineering on a 5-speed manual transmission.  It's a little shy of $3,000, but the tranny has a very low 1st gear (3.37) and 2nd is 1.99.  So, it is not a close ratio setup.  They had a better combination, but don't produce it any more.  One of their engineers indicated that I wouldn't like the current combination. My current 1st gear is 2.64 and that is a really low starting ratio with the 3.70 rear end.  My car is easily driven at 20 mph in 4th gear.  The engine doesn't lug. 

Hey, I thought this was supposed to be a friendly site?!?  I just found out that my newly acquired car is a rare one.  Don't think the previous owner realized that.  He only drove the car 3000 miles in 24 years, so he probably wasn't concerned about drivability.  I was just seeking advice from others that may have had the same questions and some possible solutions. 


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Re: change differential ratio (27/46)
 11/16/12 2:27pm
Adams' AppleLifetime Member
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Duncanville, TX - USA

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#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas


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Ok folks....before this gets any further, let's just all try to be helpful without being sarcastic, or anything like that. It's hard to tell the emotion behind printed(typed) words, and sometimes it looks like someone is talking trash, or talking down to someone. I hope that is not the case here. Most of our long time members do joke around, and talk a little smack to each other every once and a while, but it is all in fun. We want to keep it that way for everyone.Hug

Dan, you can get the diff ratio code from the bottom of the diff. That would at least tell ya what it was originally built with, assuming it has not been changed....a lot of them have been over the years. If it does indeed have a 3.70 in it now, then I think going to the 3.36 would be just about perfect for what you are looking for, RPM-wise. My 3.36 & 225/70R15s runs just shy of 3000 rpms @70mph.

http://s77.beta.photobucket.com/user/aapple2/media/brakes/Differential/codes.jpg.html?sort=3&o=1





|UPDATED|11/16/2012 11:27:35 AM (AZT)|/UPDATED|


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Re: change differential ratio (28/46)
 11/16/12 2:01pm
myhigh76
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Traverse City, MI - USA

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1976 coupe 35K miles


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I have the same concern with my '76 (38 K miles) The engine feels like it labors so much in 4th. gear at 60-65 mph.  I also am interested in driving this car a distance without feeling I need to shift into another gear...I know very little 'bout gear ratios but would like to get more answers ...Not certain what gear I am running currently. Happy to see someone with the same dilemma.  Any suggestions are appreciated!!


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Re: change differential ratio (29/46)
 11/16/12 6:56pm
tonytheroofer
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Rochester, NY - USA

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The gears of a std 4spd are 1.00 in 4th ,, same with all the automatics 1.00 in 3rd .. 

I have the 1980 with a 3.55 rear and at 70 or so i was at 3200 plus or minus a little and that also was not in my opinion a good place to run the engine day in and day out , so I would drive 55 and be the corvette every one passed ha ha ha ha ha ..................

The solution for me was the 700R4 including a new cross member and most importantly a new shifter detent that not only gave correct access to all 4 forward gears but also gave me the correct lens readings so when you look down at it you see  R N D 3 2 1. It looks so stock only us corvette nuts will notice and I bet ya people who see it will ask is this an 82 ?

Is it factory original ? NO , but the sad fact  is unless it is a 0 mile or 50 mile original its value today and for the foreseeable future is low . ( c-3 low 5k high maybe 15k ) Low mileage heated storage never touched museum piece 25k to 35k and that is not going to apply to a 77 or 80 .. 

With the 700R4 My first gear is now 3.06 when multiplied by rear ratio I am getting a 10.8 t/q multiplier that a good street car would want if it seeks decent performance, and I am  now 30% less rpm to go 70 add the lock up converter and I drop 100 or so more rpm when on the highway. Making the car a dream to drive and I have to be careful because I am so use to the revving noise I tend to keep my foot in it and realize I am at 90 lol... 

So it is all a compromise , even original numbers matching can be maintained by storing the th350 in a crate wrapped in plastic .. I long ago loving the body style decided power and good street manners trumped all else yet also felt the car should look correct to observers and even under my hood it should be less flash and more oem in how it looks. 

430hp
430tq
3.55 rear 

And to look at it and here it one would be hard pressed to suggest any serious changes have ever been made and I am proud of that . 


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Re: change differential ratio (30/46)
 11/17/12 11:22am
dapperdan
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COLUMBUS, NC - USA

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You can check the differential's gear ratio by marking the drive shaft and have someone turn a back wheel one complete turn, while you count the number of times that the drive shaft turns.  That is a very simple way of determining the gear ratio.  Most of the members on this site have recommended the 3.36 differential for those with a 3.70.

I have talked to several different suppliers and they won't make a clear recommendation, understandably.  However, one of these suppliers indicated that owners have even gone to the 3.08 to quiet down the RPMs.  That would make the car much better at highway speeds, but would affect the "hole shot."  

I am going to have a different differential put into my '77.  I will keep the site posted on what I decide upon, but it is most probably going to be the 3.36.  The speedometer gear has to be changed.  That is a simple change and the correct gear can be acquired from Ecklers or several other suppliers.


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Re: change differential ratio (31/46)
 11/17/12 2:37pm
Case75
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, - Canada

Vette(s):
1975 Corvette 383 stroker w/Patriot 190 heads, Eagle stroker kit, Comp 279TH7 cam, DUI distributor, Edelbrock 7116 intake, Edelbrock 1806 carb, Hedman Hedders, TH400 w/TCI Breakaway 2400 stall, Transgo 1-2 shift kit.


Joined: 6/19/2012
Posts: 230

tonytheroofer said: The gears of a std 4spd are 1.00 in 4th ,, same with all the automatics 1.00 in 3rd .. 

I have the 1980 with a 3.55 rear and at 70 or so i was at 3200 plus or minus a little and that also was not in my opinion a good place to run the engine day in and day out , so I would drive 55 and be the corvette every one passed ha ha ha ha ha ..................

The solution for me was the 700R4 including a new cross member and most importantly a new shifter detent that not only gave correct access to all 4 forward gears but also gave me the correct lens readings so when you look down at it you see  R N D 3 2 1. It looks so stock only us corvette nuts will notice and I bet ya people who see it will ask is this an 82 ?

Is it factory original ? NO , but the sad fact  is unless it is a 0 mile or 50 mile original its value today and for the foreseeable future is low . ( c-3 low 5k high maybe 15k ) Low mileage heated storage never touched museum piece 25k to 35k and that is not going to apply to a 77 or 80 .. 

With the 700R4 My first gear is now 3.06 when multiplied by rear ratio I am getting a 10.8 t/q multiplier that a good street car would want if it seeks decent performance, and I am  now 30% less rpm to go 70 add the lock up converter and I drop 100 or so more rpm when on the highway. Making the car a dream to drive and I have to be careful because I am so use to the revving noise I tend to keep my foot in it and realize I am at 90 lol... 

So it is all a compromise , even original numbers matching can be maintained by storing the th350 in a crate wrapped in plastic .. I long ago loving the body style decided power and good street manners trumped all else yet also felt the car should look correct to observers and even under my hood it should be less flash and more oem in how it looks. 

430hp
430tq
3.55 rear 

And to look at it and here it one would be hard pressed to suggest any serious changes have ever been made and I am proud of that . 

More and more I am beginning to think this is the best option fo me. My L48 has the original THM400; I have looked at a Keisler RS500 kit as I wish my car had a manual. It can be converted with their kit very well but it gets expensive. I've been looking at a Bowtie Overdrive conversion with the 700R4, the crossmember, TV cable and shifter and the price is more reasonable with nearly similar performance/mileage gains. I have the 3.08 rear; my THM400 has a 2.48 first vs the 700R4 with the 3.06 first. This will give me the equivalent hole shot of a 3.80 rear gear with the .70 overdrive netting a 2.16 highway ratio. Best of both worlds I think. I agree that my '75 will only increase in value about equal to inflation, maybe a bit better. I bought the car for me to enjoy and getting lousy gas mileage is keeping some of the enjoyment out of it.

|UPDATED|11/17/2012 11:37:41 AM (AZT)|/UPDATED|


______________
 
Build Date: May 7, 1975. 383 w/267 RWHP/310 RWTQ
Re: change differential ratio (32/46)
 11/18/12 1:00pm
ROCKRDR
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Lakeside, AZ - USA

Vette(s):
1975 C3 Stingray


Joined: 7/23/2012
Posts: 126

Case75 said:
tonytheroofer said: The gears of a std 4spd are 1.00 in 4th ,, same with all the automatics 1.00 in 3rd .. 

I have the 1980 with a 3.55 rear and at 70 or so i was at 3200 plus or minus a little and that also was not in my opinion a good place to run the engine day in and day out , so I would drive 55 and be the corvette every one passed ha ha ha ha ha ..................

The solution for me was the 700R4 including a new cross member and most importantly a new shifter detent that not only gave correct access to all 4 forward gears but also gave me the correct lens readings so when you look down at it you see  R N D 3 2 1. It looks so stock only us corvette nuts will notice and I bet ya people who see it will ask is this an 82 ?

Is it factory original ? NO , but the sad fact  is unless it is a 0 mile or 50 mile original its value today and for the foreseeable future is low . ( c-3 low 5k high maybe 15k ) Low mileage heated storage never touched museum piece 25k to 35k and that is not going to apply to a 77 or 80 .. 

With the 700R4 My first gear is now 3.06 when multiplied by rear ratio I am getting a 10.8 t/q multiplier that a good street car would want if it seeks decent performance, and I am  now 30% less rpm to go 70 add the lock up converter and I drop 100 or so more rpm when on the highway. Making the car a dream to drive and I have to be careful because I am so use to the revving noise I tend to keep my foot in it and realize I am at 90 lol... 

So it is all a compromise , even original numbers matching can be maintained by storing the th350 in a crate wrapped in plastic .. I long ago loving the body style decided power and good street manners trumped all else yet also felt the car should look correct to observers and even under my hood it should be less flash and more oem in how it looks. 

430hp
430tq
3.55 rear 

And to look at it and here it one would be hard pressed to suggest any serious changes have ever been made and I am proud of that . 

More and more I am beginning to think this is the best option fo me. My L48 has the original THM400; I have looked at a Keisler RS500 kit as I wish my car had a manual. It can be converted with their kit very well but it gets expensive. I've been looking at a Bowtie Overdrive conversion with the 700R4, the crossmember, TV cable and shifter and the price is more reasonable with nearly similar performance/mileage gains. I have the 3.08 rear; my THM400 has a 2.48 first vs the 700R4 with the 3.06 first. This will give me the equivalent hole shot of a 3.80 rear gear with the .70 overdrive netting a 2.16 highway ratio. Best of both worlds I think. I agree that my '75 will only increase in value about equal to inflation, maybe a bit better. I bought the car for me to enjoy and getting lousy gas mileage is keeping some of the enjoyment out of it.



I still need to see what my rear ratio is as well, I'm thinking the 700R4 conversion as well
has anyone done this yet...on a budget :) the trannys are pretty cheap on ebay. someone 
has one out here for 300.00 but I dont know what im looking for in a tranny.
I  guess this might be subject for a diff topic.



______________
1975 Corvette Coupe 
My first Vette.....trying to fix stuff

My Project:
https://docs.google.com/folder/d/0B6uOAIjGf3cWejQ5TjNkcVRybGs/edit
Re: change differential ratio (33/46)
 11/18/12 2:41pm
Case75
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, - Canada

Vette(s):
1975 Corvette 383 stroker w/Patriot 190 heads, Eagle stroker kit, Comp 279TH7 cam, DUI distributor, Edelbrock 7116 intake, Edelbrock 1806 carb, Hedman Hedders, TH400 w/TCI Breakaway 2400 stall, Transgo 1-2 shift kit.


Joined: 6/19/2012
Posts: 230

ROCKRDR said:
Case75 said:
tonytheroofer said: The gears of a std 4spd are 1.00 in 4th ,, same with all the automatics 1.00 in 3rd .. 

I have the 1980 with a 3.55 rear and at 70 or so i was at 3200 plus or minus a little and that also was not in my opinion a good place to run the engine day in and day out , so I would drive 55 and be the corvette every one passed ha ha ha ha ha ..................

The solution for me was the 700R4 including a new cross member and most importantly a new shifter detent that not only gave correct access to all 4 forward gears but also gave me the correct lens readings so when you look down at it you see  R N D 3 2 1. It looks so stock only us corvette nuts will notice and I bet ya people who see it will ask is this an 82 ?

Is it factory original ? NO , but the sad fact  is unless it is a 0 mile or 50 mile original its value today and for the foreseeable future is low . ( c-3 low 5k high maybe 15k ) Low mileage heated storage never touched museum piece 25k to 35k and that is not going to apply to a 77 or 80 .. 

With the 700R4 My first gear is now 3.06 when multiplied by rear ratio I am getting a 10.8 t/q multiplier that a good street car would want if it seeks decent performance, and I am  now 30% less rpm to go 70 add the lock up converter and I drop 100 or so more rpm when on the highway. Making the car a dream to drive and I have to be careful because I am so use to the revving noise I tend to keep my foot in it and realize I am at 90 lol... 

So it is all a compromise , even original numbers matching can be maintained by storing the th350 in a crate wrapped in plastic .. I long ago loving the body style decided power and good street manners trumped all else yet also felt the car should look correct to observers and even under my hood it should be less flash and more oem in how it looks. 

430hp
430tq
3.55 rear 

And to look at it and here it one would be hard pressed to suggest any serious changes have ever been made and I am proud of that . 

More and more I am beginning to think this is the best option fo me. My L48 has the original THM400; I have looked at a Keisler RS500 kit as I wish my car had a manual. It can be converted with their kit very well but it gets expensive. I've been looking at a Bowtie Overdrive conversion with the 700R4, the crossmember, TV cable and shifter and the price is more reasonable with nearly similar performance/mileage gains. I have the 3.08 rear; my THM400 has a 2.48 first vs the 700R4 with the 3.06 first. This will give me the equivalent hole shot of a 3.80 rear gear with the .70 overdrive netting a 2.16 highway ratio. Best of both worlds I think. I agree that my '75 will only increase in value about equal to inflation, maybe a bit better. I bought the car for me to enjoy and getting lousy gas mileage is keeping some of the enjoyment out of it.



I still need to see what my rear ratio is as well, I'm thinking the 700R4 conversion as well
has anyone done this yet...on a budget :) the trannys are pretty cheap on ebay. someone 
has one out here for 300.00 but I dont know what im looking for in a tranny.
I  guess this might be subject for a diff topic.


In 1975, L48 automatics came with the 3.08 rear and a 2.73 rear was optional (why anyone would order this I can't imagine!). If it was an L48 manual, it came with the 3.36 gears and the 3.08 was optional. L82 autos came with 3.36 gears with 3.55 being optional. L82 4 speed wide ratios came only with 3.55 gears while close ratios had 3.55 std and 3.70 optional. :)

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Build Date: May 7, 1975. 383 w/267 RWHP/310 RWTQ
Re: change differential ratio (34/46)
 11/18/12 5:51pm
dapperdan
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Thank you for the data on the various combinations of the '75 Corvette differentials.  Those just re-enforce my wanting to change my '77 L82 from a 3.70 to a 3.36.

I wonder how the L48 did with the 3.08 with a 4-speed???  I think that would have been a good combination for mpg, but not too good for performance.  Anybody out there have/had that combination?


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Re: change differential ratio (35/46)
 11/19/12 12:16am
ROCKRDR
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ok checked mine, numbers are AWW 318 E2 so I guess I have the 3.08 in mine

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Re: change differential ratio (36/46)
 11/19/12 12:49am
dapperdan
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How well does your car perform?  What kind of MPG do you get???

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Re: change differential ratio (37/46)
 11/20/12 1:01pm
Adams' AppleLifetime Member
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Duncanville, TX - USA

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#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas


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I'm loosing it...
somewhere in this thread(or the one on 700s), someone mentioned having the original gear set out of their car from the PO, and I mentioned how to determine the ratio of that gearset by counting the teeth, or looking at the markings. Anyway, I cannot find that post now, so I'll add the pic I was looking for at the time here instead, so ya'll will know what to look for.....



Not ALL gearsets will have the actual ratio stamped on them, as this one does....but they will all have the ring and pinion tooth counts stamped(or engraved). Thumbs Up


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Re: change differential ratio (38/46)
 11/21/12 10:15am
Jaws79Lifetime Member
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Man, better use that ratio for racing only!
 
Barry
 
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|UPDATED|11/21/2012 7:15:32 AM (AZT)|/UPDATED|


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Re: change differential ratio (39/46)
 11/21/12 1:45pm
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Or pullin stumps....or rock climbin...or just 'cause it's there....and it was free...LOL

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Re: change differential ratio (40/46)
 3/18/13 7:46pm
F4GaryGold Member
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3.36 would be perfect.  I wish I had done that.

So can I put a 2.64 first gear in my tranny if it doesn't already have one?  I assume the LT-1 came with a close ratio tranny.

As far as originality in your rear end.  Wink  I don't think the NCRS checks the actual rear end ratio.  I assume they just check the numbers stamped on the rear end housing.  Kinda like changing the internals of an original engine.  What they can't see they can't check.

Go for it.


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Re: change differential ratio (41/46)
 3/19/13 10:36am
tonytheroofer
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Resurrection of a thread :):) 

Someone said they have a 3.08 rear , if you do , you can not run an OD transmission because your carb is unable to regulate the fuel mixture for the high load low rpm situation you will encounter, Your motor will kick and buck like a steer at a rodeo ..  


My best guess for rear gears would be no less then 3.36 ,,, however a 3.55 up to a 4.11 would be a perfect range for all driving even if your in OD as early as 40mph. So if you have a really low rear and no fuel injection you can not run the OD units. 


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Re: change differential ratio (42/46)
 3/19/13 12:32pm
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F4Gary said: 3.36 would be perfect.  I wish I had done that.

So can I put a 2.64 first gear in my tranny if it doesn't already have one?  I assume the LT-1 came with a close ratio tranny.

As far as originality in your rear end.  Wink  I don't think the NCRS checks the actual rear end ratio.  I assume they just check the numbers stamped on the rear end housing.  Kinda like changing the internals of an original engine.  What they can't see they can't check.

Go for it.

Why, coitainly....all ya gotta do is change the cluster gear, and the 1st, 2nd, and 3rd gears on the mainshaft....or just swap the trans for a wide ratio and be done with it. Thumbs Up

As far as NCRS stuff on the gear ratios, it wouldn't matter at all unless you were going for a P/V(performance verification). They check the rpm vs speed, and mileage on that, so you wouldn't be able to get away with a rear ratio change.


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Re: change differential ratio (43/46)
 6/17/13 8:20am
thallow
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dan did you get the ratio changed. I am considering a change also and was wondering how it came out.  My 69 has 3.7 with a tremic 5 in it and I agree 1st gear is way to low, I start in 2nd most of the time but it is a bit tall. 70 in 5th is about 2360. The engine pulls good at the low rpm, and we use it for long trips.



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Re: change differential ratio (44/46)
 6/17/13 9:43am
dapperdan
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I have decided not to change the rear end, mostly to keep the car in its original configuration.  I just learned the it has the "Rockcrusher" transmission, so I am going to keep that too.  Had the U-joints replaced; the rear strut rods and the 4 shocks.  Also, had the inside rear wheel bearings  lubricated with a device that I got from Zip Corvette.  I am just going to live with the 3.70 rear end, since the car really runs great with the setup that the Chevy engineers thought was a good compromise at the time.  The car is a "trip" to drive, but it is just my toy, since I have a 2011 Grand Sport as my daily driver.

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Re: change differential ratio (45/46)
 8/9/13 9:00pm
RalphTilLifetime Member
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Sorry but a move to a 3:53 rear is not worth it.  If you want to get your gas mileage up say, above 20 MPG, you will need to go to a Tremec 5-Speed trans and a 3:07 rear.  We have tried every variety above that (ie 3:36, 3:55)  and you struggle to get to 20 MPG.  I, too, have a '77 and all that numbers matching stuff is nice but doesn't really move the $ needle.  I would make the changes that improve driveability and save the original parts.  Gas was under $1/gallon when these Vettes were new and the big gears helped with the smogged engine performance.  I have a Turbo 400 automatic, L82 with a 3:55 rear and it races at 70 MPH.  I'm going to a 2:87 rear with an engine rebuild over this winter.  Only has 61,000 miles but valve seals and guides are leaking.

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Re: change differential ratio (46/46)
 6/11/15 6:35pm
dwa175
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hi  I know on this site the fellas  don't like overdrives  I have used overdrive in my 49 ford to 73 chevy p/u  easy to do  takes up no room   little bit      nobody likes them.my 79 runs 3200 rpms at 60 mph hour  50 to 60 miles cost a half of tank of fuel,i have looked high and low  for a overdrive unit   no luck   contact a company that's sales overdrive  they want me to buy a new 5 speed   at 5,000 $  not going to happen,going to calf  this mouth  I will find a overdrive unit if I do  I will tell you  and you can do as you wish  good luck  dwa     p/s  can stand using that mush gas for short trip.Beer
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