Rear end ratio question...
Thread Starter
Advanced
Joined: May 2017
Posts: 79
Likes: 7
From: VA
Rear end ratio question...
I'm a new, first time Corvette owner, so I have a lot of questions. last month I bought a 1979, 4-speed, L-82 Corvette. The guy I bought it from didn't know a lot either.
My question is, your best guess, what is the rear end ratio? It is turning 3,500 rpm in fourth gear at 70 mph.
My question is, your best guess, what is the rear end ratio? It is turning 3,500 rpm in fourth gear at 70 mph.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
That would be a 4.11:1 or thereabouts. I suggest you verify the ratio by getting the rear of the car up on jacks and make one revolution of the tire and count the revolutions of the driveshaft. If it is a 4.11, then the driveshaft would turn a nick over four revolutions. That wouldn't be a factory ratio and would have been installed sometime in the car's life. If the visual method gives you a different result, like 3.5 revolutions in the driveshaft, then you know your tach is out of whack.
Instructor
Joined: Jun 2015
Posts: 178
Likes: 34
From: Calgary Alberta
I'm a new, first time Corvette owner, so I have a lot of questions. last month I bought a 1979, 4-speed, L-82 Corvette. The guy I bought it from didn't know a lot either.
My question is, your best guess, what is the rear end ratio? It is turning 3,500 rpm in fourth gear at 70 mph.
My question is, your best guess, what is the rear end ratio? It is turning 3,500 rpm in fourth gear at 70 mph.
Le Mans Master
Joined: Dec 2007
Posts: 6,422
Likes: 591
From: McHenry Illinois
If the tach is accurate and you have a 26"tire, then a 3.91 diff will be turning around 3500 in 4th.
https://www.ringpinion.com/calculators/Calc_RPM.aspx
https://www.ringpinion.com/calculators/Calc_RPM.aspx
Last edited by '75; Jun 10, 2017 at 04:50 PM.
Melting Slicks
Joined: Sep 2004
Posts: 3,223
Likes: 666
From: Northeast
In 79, L82, whether you got the wide ratio transmission or close ratio the base rear end was 3.70. With an option for the 3.36. If no one has been in there then probably you have 3.70 and the tach not accurate.
Crawl under and determine for yourself because someone along the line could have changed it.
If you are interested in obtaining the original invoice for the car, contact these people and for $50 they will send you a copy of the invoice. Doesnt mean that is what is on there now but shipped from the factory:
https://www.gmheritagecenter.com/gm-...tion-kits.html
If you scroll down about 1/2 way and click on corvette "show", then 79, you will get a 133 page pdf file which is some good reading.
Crawl under and determine for yourself because someone along the line could have changed it.
If you are interested in obtaining the original invoice for the car, contact these people and for $50 they will send you a copy of the invoice. Doesnt mean that is what is on there now but shipped from the factory:
https://www.gmheritagecenter.com/gm-...tion-kits.html
If you scroll down about 1/2 way and click on corvette "show", then 79, you will get a 133 page pdf file which is some good reading.
Last edited by BKarol; Jun 10, 2017 at 05:17 PM.
Thread Starter
Advanced
Joined: May 2017
Posts: 79
Likes: 7
From: VA
Ok, great help everyone! I do have 225/70-15 tires. The engine does have a lot of non-stock stuff, so perhaps the rear gears were changed also. I will take y'alls advise, get it on a lift, and turn the rear tire count the driveshaft turns. Thanks again, love this forum! 
Thread Starter
Advanced
Joined: May 2017
Posts: 79
Likes: 7
From: VA
In 79, L82, whether you got the wide ratio transmission or close ratio the base rear end was 3.70. With an option for the 3.36. If no one has been in there then probably you have 3.70 and the tach not accurate.
Crawl under and determine for yourself because someone along the line could have changed it.
If you are interested in obtaining the original invoice for the car, contact these people and for $50 they will send you a copy of the invoice. Doesnt mean that is what is on there now but shipped from the factory:
https://www.gmheritagecenter.com/gm-...tion-kits.html
If you scroll down about 1/2 way and click on corvette "show", then 79, you will get a 133 page pdf file which is some good reading.
Crawl under and determine for yourself because someone along the line could have changed it.
If you are interested in obtaining the original invoice for the car, contact these people and for $50 they will send you a copy of the invoice. Doesnt mean that is what is on there now but shipped from the factory:
https://www.gmheritagecenter.com/gm-...tion-kits.html
If you scroll down about 1/2 way and click on corvette "show", then 79, you will get a 133 page pdf file which is some good reading.
Thanks karol! A lot of great info on that pdf including the fact that the L-82 with a 4-speed had a 3.70:1 axle ratio. So now I know!
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Le Mans Master
Joined: Oct 2007
Posts: 7,445
Likes: 971
From: Tennessee/Rhode Island
C3 electronic tachs (75-82) and the speedometers are notoriously inaccurate. I know for sure that my 78 tach reads 300 rpm HIGH at any given mid to high RPM, verified by 2 dyno events and the engine builder who did the bottom end of the rebuilt/upgraded 355 L-82.
With my 4 speed and 3.70 gearing at 3,500 indicated rpm, my speed is also 70 MPH...I have 26.0 inch diameter rear tires and definitely 3.70 stock L-82 4 speed gears in the tranny diff. The speedometer should indicate 70 mph @ 3,200 rpm with my 3.70 gears and 26.0 inch tires.
My guess is that you do have 3.70 gears and either the tach or the speedometer is incorrect like on mine.
When I shift at 6,000 RPM on the tach, I know that the actual revs is 5,700 RPM which is a nice cushion to have to protect the engine.....
Last edited by jb78L-82; Jun 19, 2017 at 06:07 AM.