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4/12/14 10:02pm - Original Message: 'Gear Ratios'
Feet
Former Member
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Grantsville, UT - USA

Vette(s):
1978 25th Anniversary 350, 4spd

Joined: 7/11/2013
Posts: 6

At some point it my 1978 Silver Anniversary's life it was converted from an automatic to a 4spd, I don't care because I just want to have a nice driver not real worried about the numbers matching and I enjoy driving a standard but my rpm's are high in 4th at highway speeds  60mph at about 3500 rpm. I have not checked the rear end ratio yet but would leaving the ratio for the automatic cause this problem.  Any other suggestions!

Thanks


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4/12/14 11:54pm - Reply: 'Re: Gear Ratios'
HighDesertDave
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Carson City, NV - USA

Vette(s):
Numbers matching 1972 350 4 speed, silver pewter body with blue interior

Joined: 12/4/2013
Posts: 239
I think your gears are non OE. 3500 RPM at 60 MPH would suggest 4.10 gears. My 4 spd with 3.36 gears tachs around 2600 @ 60 MPH. My 57 BelAir with 4 spd tachs 3100 RPM with 3.73 gears at 60 MPH. Similar size tires.
Dave

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My Christmas present to ME! Bought Sat 11-30-2013. 72, 350 4 spd, Silver with blue interior...Can't you just hear Pablo Cruz playing in the background?

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4/13/14 11:03am - Reply: 'Re: Gear Ratios'
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: 19545
The car prolly has a 3.55 rear ratio if it had the automatic. There was a 3.70 ratio available, but only for the 4 speed cars. The change in trans from auto to 4 speed does NOT change anything as far as rpms x mph on the road, since both 4 speed and automatic is the same high gear ratio...1:1, or direct.
You can look at the numbers on the bottom of the diff to see what it originally had, but that's no guarantee that the gears have not been changed somewhere down the line.
I'd guess the car still has the original 3.55 ratio, but who knows what has been done before you got it. Unhappy

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Joel Adams
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4/13/14 2:19pm - Reply: 'Re: Gear Ratios'
HighDesertDave
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Carson City, NV - USA

Vette(s):
Numbers matching 1972 350 4 speed, silver pewter body with blue interior

Joined: 12/4/2013
Posts: 239
Here's a good engine RPM calculator you can plug your numbers into. I send this to my customers when they are trying to pic a gear for our rear ends.  Click HERE...

If I recall the stock Turbo 350/400 torque converter runs at about 85-90% efficiency which relates to something like a 200 RPM higher rate with the same rear end ratio (compared to a 4spd) due to slippage. The above calculator compensates for this. You can jack the rear end of the car up, have someone turn your rear wheel exactly 1 turn by hand while you count the times your drive shaft turns over. You want to mark it with a fixed ref point off to the side of the drive shaft so when your down to that .36 .55 .70 .10 of your last turn you can figure your gear. Its doable....for example the dif between .36 and .55 is 68 degrees of drive shaft turn. You can visually see this with masking tape and sharpie marks. Hope that makes sense!
Both tires will need to be off the ground with a posi....turn the wheel forward.




____________________________________
My Christmas present to ME! Bought Sat 11-30-2013. 72, 350 4 spd, Silver with blue interior...Can't you just hear Pablo Cruz playing in the background?

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4/13/14 2:35pm - Reply: 'Re: Gear Ratios'
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: 831
I can't speak for the autos, but there is a reason that for many years the 3.36 ratio came standard with the L-48 4 speed cars.  It is a good all around ratio that does not have you screaming along just going 60, but still decent enough off the line.  I wouldn't dream of changing it considering I've rebuilt for a pretty good bump in power (alleviating the need for lower gears), I plan to keep my original borg-warner (vs putting a 5 speed OD in for a lot of money) and I use this car as a cruiser vs a drag racer.   

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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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4/13/14 4:38pm - Reply: 'Re: Gear Ratios'
JohnRR
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Joined: 2/22/2011
Posts: 110
What's also helpful, is knowing what 4-speed trans you have.  Hopefully, you have the Muncie wide ratio trans with the lower 1st. gear (2.52).  If you have the close ratio (2.20 first) then you might want to consider nothing numerically less than 3.55 ratio rear gears.  

Do you want to check the rear end gear ratio in your garage?  It's pretty easy.

John



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1973 BB 4-Speed Coupe Project Car
2009 Z06 Mostly Stock
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4/13/14 5:23pm - Reply: 'Re: Gear Ratios'
VETTERBOB Lifetime Member
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DUNEDIN, FL - USA

Vette(s):
1969 COUPE CORTEZ SILVER, BRIGHT BLUE INTERIOR 383 cu. in. 512 HP. Hooker headers w/ side pipes PS PS AC AM FM STEREO. 68,000.MILES C5 5oth Anniversary Coupe , auto.350hp

Joined: 10/22/2004
Posts: 827
Definitely 3:55.i run them with my T 400 auto. Rpm matches.

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I might be going to hell in a bucket, but at least I`m enjoying the ride!

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4/13/14 7:40pm - Reply: 'Re: Gear Ratios'
Feet
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Grantsville, UT - USA

Vette(s):
1978 25th Anniversary 350, 4spd

Joined: 7/11/2013
Posts: 6

Thanks for all the help, looks like I will be looking for a new ring and pinion or complete 3rd member after finding out what I really have. Maybe a trade for someone who wants something lower in their project.

Thanks again

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4/14/14 6:28pm - Reply: 'Re: Gear Ratios'
F4Gary Gold Member
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Grapevine, TX - USA

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

Joined: 8/26/2006
Posts: 943
You have a 4.56 rear end.

http://www.prosystemsracing.com/calculate.html


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4/14/14 8:04pm - Reply: 'Re: Gear Ratios'
HighDesertDave
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Carson City, NV - USA

Vette(s):
Numbers matching 1972 350 4 speed, silver pewter body with blue interior

Joined: 12/4/2013
Posts: 239
I think we should place some bets Wink . We be all over the board....

____________________________________
My Christmas present to ME! Bought Sat 11-30-2013. 72, 350 4 spd, Silver with blue interior...Can't you just hear Pablo Cruz playing in the background?

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4/14/14 9:33pm - Reply: 'Re: Gear Ratios'
JohnRR
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Joined: 2/22/2011
Posts: 110
Ok Feet, here's how to check the rear end gear ratio:

Get rear of your Vette up on two jack stands.

Place the trans in Neutral.

Put chalk marks at the 6:00 position on the inside of your rear tires.

Put a chalk mark on the bottom of your rear U-Joint or driveshaft.

Then, by hand, carefully rotate the driveshaft and count the turns it makes until the rear tires make exactly one revolution. 

The number of turns of your driveshaft will equal your rear end gear ratio.

Is this something you can do?

John

BTW, if you really have a 4.56 rear gear ratio, you should be able to take off from a dead stop in 3rd. or 4th gear without too much trouble.  I had a 4.88 rear for awhile and 4th gear was like an "All purpose gear".  The first 3 gears were not all that necessary for just cruising. 





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1973 BB 4-Speed Coupe Project Car
2009 Z06 Mostly Stock
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4/14/14 9:46pm - Reply: 'Re: Gear Ratios'
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: 19545
VETTERBOB said: Definitely 3:55.i run them with my T 400 auto. Rpm matches.


I think that should tell the tale....Thumbs Up
Without knowing the tire diameter, there's no way to calculate the ratio from just RPM/MPH. Jacking it up and spinning the tires, and counting the driveshaft revolutions will get you just about as close as you can without actually looking at the gear set.
There are only a certain number of ratios that will fit into the stock(original) diff housing. In this case, I think the 3.55 is all that would work, without changing the diff carrier. The 3.70 and 4.11 ratios that were available for the year took a specific carrier that would only be used for those two ratios. However, we also do not know if any of the stuff has been changed, so really, it's a crap shoot, and we're all just guessing....well...everyone except Bob, who happens to have the 3.55 gears, and turns the same RPMS at the same MPH....Wink




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Joel Adams
C3VR Lifetime Member #56    
My Link


(click for Texas-sized view!)
             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
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4/15/14 4:35am - Reply: 'Re: Gear Ratios'
Jeffm
Former Member
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Sebago Lake, ME - USA

Vette(s):
1978 Silver Anniversary L82 4spd (M21) Red Leather. Pace Car is in the hands of a new caretaker.

Joined: 2/15/2007
Posts: 964
My stock Silver Ann  and the Pace car both with the L-82, 4 speed and 3.70 tachs 3500 somewhere between 60 -65. I'll pay closer attention next time I'm out.
Maybe you got the 4.10 if doing 3500 right at 60mph. Converting to 4 spd they may have wanted that extra get up and go. 


|UPDATED|4/15/2014 2:35:25 AM (AZT)|/UPDATED|


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4/16/14 2:08am - Reply: 'Re: Gear Ratios'
cthulhu Lifetime Member
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Hot Springs, AR - USA

Vette(s):
69 Conv #'s Match 427, TKO-600, 3.36HD-Posi, T/T, PS, PB, PW, SP, Lthr, Comp Mag 270 & Pro Rlr Rockers, Edel Air-Gap RPM, BG SD 750 MS, MSD 6AL+Tach Dist+ Blaster SS+Wires, Hly Bl Fuel Pump, K&N, Jet-Hot Hkr Side-Pipes,Steeroids, Al Rad, Spal Fans

Joined: 4/24/2004
Posts: 3194
JohnRR said: Ok Feet, here's how to check the rear end gear ratio:

Get rear of your Vette up on two jack stands.

Place the trans in Neutral.

Put chalk marks at the 6:00 position on the inside of your rear tires.

Put a chalk mark on the bottom of your rear U-Joint or driveshaft.

Then, by hand, carefully rotate the driveshaft and count the turns it makes until the rear tires make exactly one revolution. 

The number of turns of your driveshaft will equal your rear end gear ratio.

Is this something you can do?

John

BTW, if you really have a 4.56 rear gear ratio, you should be able to take off from a dead stop in 3rd. or 4th gear without too much trouble.  I had a 4.88 rear for awhile and 4th gear was like an "All purpose gear".  The first 3 gears were not all that necessary for just cruising. 






Adding a little more precision Make a chalk mark on the driver shaft where it stops Using a sewing tape to measure the distance between the two marks and divide by the circumference to get the fractional portion. Example. If the the circumference is 5". And your final mark is 1.9" past your starting mark you would have a 3.36, 2.6 =3.55, 3.5"= 3.70. Actually even better in centimeters.

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