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8/19/12 3:17pm - Original Message: 'change differential ratio'
dapperdan
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COLUMBUS, NC - USA

Vette(s):
2011 Grand Sport Coupe, 6-speed 1977 Coupe L82, 4-speed

Joined: 8/19/2012
Posts: 42

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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8/19/12 3:49pm - Reply: 'Re: change differential ratio'
DStoffel
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Wampum, PA - USA

Vette(s):
1999 coupe Magnetic Red /w grey interior 30,000 miles.

Joined: 8/27/2008
Posts: 90
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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8/19/12 7:58pm - Reply: 'Re: change differential ratio'
corvette440hp Lifetime Member
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Joined: 7/2/2006
Posts: 8205
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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8/20/12 1:21am - Reply: 'Re: change differential ratio'
rod7515 Lifetime Member
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Red Lion, PA - USA

Vette(s):
1974 White 350 Corvette, TH400 Automatic 1972 Tangerine /Go Mango Convertible 383 Stroker, 2004r Automatic

Joined: 10/27/2006
Posts: 1172
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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8/20/12 1:53am - Reply: 'Re: change differential ratio'
daveo76
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Vancouver, WA - USA

Vette(s):
1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.

Joined: 8/25/2005
Posts: 829
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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1976 Silver/Firethorn.  L48, 4spd.  Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.
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8/20/12 8:09am - Reply: 'Re: change differential ratio'
DaveM Lifetime Member
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Flanders, NJ - USA

Vette(s):
1982 Black, ZZ4, Hooker Side Pipes, 3.55 Rear, Rebuilt & upgraded 700R4, All new suspension, brakes, & new Charcoal interior.

Joined: 1/27/2004
Posts: 2530
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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8/20/12 11:39am - Reply: 'Re: change differential ratio'
dapperdan
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COLUMBUS, NC - USA

Vette(s):
2011 Grand Sport Coupe, 6-speed 1977 Coupe L82, 4-speed

Joined: 8/19/2012
Posts: 42
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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8/20/12 1:40pm - Reply: 'Re: change differential ratio'
Adams' Apple Lifetime Member
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Duncanville, TX - USA

Vette(s):
#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

Joined: 11/8/2003
Posts: 19447
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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8/20/12 3:03pm - Reply: 'Re: change differential ratio'
dskopp
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Oak Creek, WI - USA

Vette(s):
1981 Great White Shark. Red Interior, 350/190hp. PS, PB (SS), A/C CC, T-Tops, Going to remain as Stock as possible. Served three years in Active Duty Army, then Retired Air Force after 34 years! Badger State Vetts Car Club. 175,000 Original miles!!

Joined: 5/21/2008
Posts: 1956
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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8/20/12 11:11pm - Reply: 'Re: change differential ratio'
F4Gary Gold Member
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Grapevine, TX - USA

Vette(s):
1972 LT-1 convertible with factory air.

Joined: 8/26/2006
Posts: 862
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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8/21/12 2:24am - Reply: 'Re: change differential ratio'
daveo76
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Vancouver, WA - USA

Vette(s):
1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.

Joined: 8/25/2005
Posts: 829
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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1976 Silver/Firethorn.  L48, 4spd.  Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.
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8/21/12 8:15am - Reply: 'Re: change differential ratio'
manchestershark Lifetime Member
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Equinunk, PA - USA

Vette(s):
1972 conv, 4-speed, 350, 200hp, numbers match, rally wheels, war bonnett yellow w/white top. good condition, nice driver.

Joined: 10/31/2007
Posts: 1895
 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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8/21/12 2:32pm - Reply: 'Re: change differential ratio'
dapperdan
Former Member
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COLUMBUS, NC - USA

Vette(s):
2011 Grand Sport Coupe, 6-speed 1977 Coupe L82, 4-speed

Joined: 8/19/2012
Posts: 42
How tough was it to do the removal and install of the rear end?

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8/21/12 2:35pm - Reply: 'Re: change differential ratio'
dapperdan
Former Member
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COLUMBUS, NC - USA

Vette(s):
2011 Grand Sport Coupe, 6-speed 1977 Coupe L82, 4-speed

Joined: 8/19/2012
Posts: 42
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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8/21/12 2:38pm - Reply: 'Re: change differential ratio'
dapperdan
Former Member
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COLUMBUS, NC - USA

Vette(s):
2011 Grand Sport Coupe, 6-speed 1977 Coupe L82, 4-speed

Joined: 8/19/2012
Posts: 42
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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8/21/12 2:39pm - Reply: 'Re: change differential ratio'
dapperdan
Former Member
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COLUMBUS, NC - USA

Vette(s):
2011 Grand Sport Coupe, 6-speed 1977 Coupe L82, 4-speed

Joined: 8/19/2012
Posts: 42
OK, Thanks for the hello!

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8/21/12 2:41pm - Reply: 'Re: change differential ratio'
dapperdan
Former Member
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COLUMBUS, NC - USA

Vette(s):
2011 Grand Sport Coupe, 6-speed 1977 Coupe L82, 4-speed

Joined: 8/19/2012
Posts: 42
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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8/21/12 2:43pm - Reply: 'Re: change differential ratio'
dapperdan
Former Member
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COLUMBUS, NC - USA

Vette(s):
2011 Grand Sport Coupe, 6-speed 1977 Coupe L82, 4-speed

Joined: 8/19/2012
Posts: 42
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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8/21/12 2:44pm - Reply: 'Re: change differential ratio'
dapperdan
Former Member
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COLUMBUS, NC - USA

Vette(s):
2011 Grand Sport Coupe, 6-speed 1977 Coupe L82, 4-speed

Joined: 8/19/2012
Posts: 42
I will proly go with the 3.36.  Thanks for the advice.

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8/21/12 2:52pm - Reply: 'Re: change differential ratio'
JohnRR
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Joined: 2/22/2011
Posts: 110
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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1973 BB 4-Speed Coupe Project Car
2009 Z06 Mostly Stock
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