Rear Gear Ratio
Thread Starter
Instructor
Joined: Apr 2006
Posts: 201
Likes: 3
Rear Gear Ratio
Hello Folks, I'm trying to figure out what the rear gear ratio is in my 1970 Vette. I cleaned it off a little bit and found these numbers. Can anyone interpret them or do they identify the ratio. This first set of numbers is inscribed in the bottom edge of the bottom bolt flange.
I believe what I can tell is: 1 CAV 4 24 70 W. the 2nd digit might be a G instead of C. Then, cast into the diff housing on the right side bolt flange just forward of the fill plug I found this letter/number: C240. It might be O240. Do these help to identify the diff and the ratio. Thanks, Dave.
I believe what I can tell is: 1 CAV 4 24 70 W. the 2nd digit might be a G instead of C. Then, cast into the diff housing on the right side bolt flange just forward of the fill plug I found this letter/number: C240. It might be O240. Do these help to identify the diff and the ratio. Thanks, Dave.
Race Director
Joined: Aug 1999
Posts: 18,126
Likes: 174
From: WI
If you have the car up in the air high enough to read the bottom of the diff then turn a wheel one turn and count the revolutions of the driveshaft, a piece of your kids chalk helps as does the kid to climb under there and help. BTW a "CAV" diff started life as a 3:08
Last edited by redvetracr; Dec 10, 2014 at 07:37 PM.
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,754
From: Charlotte NC
AND....Being so old...ASSUMING that in the cars life the gear has not been changed.....which occurs.
DUB
Thread Starter
Instructor
Joined: Apr 2006
Posts: 201
Likes: 3
Rear Gear Ratio
Yep, thanks. I did count the driveshaft turns to one rev of the wheel. For years I thought this thing had a 3:36 gear. But in counting it today I got to 3 turns of the shaft and then when the wheel got back to its one turn starting point the shaft did not make it to even a 1/4 turn more. I thought, hmm, that ain't .36. Did it again and came up with the same thing, just a smidgen past 3 turns of the driveshaft. That's when I started looking for numbers. Started thinking it must be a 3:08. Just looking for confirmation now. Thanks again.
I'm going to put a new engine in this winter and needed to know what I was working with. Dave.
I'm going to put a new engine in this winter and needed to know what I was working with. Dave.
2026 Loser of the Year
Joined: Sep 2013
Posts: 36,599
Likes: 7,046
From: New Or-leens Loo-z-anna
Do you remember the tachometer rpm reading at 60 mph?
For example, my '68 turns 3,000 rpm at 60mph with stock diameter tires........that would equal about 3.36 gears.
It does appear that you have 3.08 gears which would be good for long trips and better mpg.
For example, my '68 turns 3,000 rpm at 60mph with stock diameter tires........that would equal about 3.36 gears.
It does appear that you have 3.08 gears which would be good for long trips and better mpg.
Thread Starter
Instructor
Joined: Apr 2006
Posts: 201
Likes: 3
If I remember right I believe I am turning about 2700 rpms at 65mph. I had a formula for rpms to mph but I might not have written it down right. It was: Mph = tire dia x rpms / axle ratio. If I transpose that for axle ratio it would look something like this:
axle ratio = tire dia. x rpms / mph. but no matter what I plug into those values it doesn't come out right. So skip that idea.
I'm thinking it is a 3:08 now too. But here's another brain teaser. If you turn the wheel one revolution and count the turns of the drive shaft, that seems to work with a posi. But if you do that with a non-posi it doesn't work. It doesn't come out anywhere near right. What gives? Dave.
axle ratio = tire dia. x rpms / mph. but no matter what I plug into those values it doesn't come out right. So skip that idea.
I'm thinking it is a 3:08 now too. But here's another brain teaser. If you turn the wheel one revolution and count the turns of the drive shaft, that seems to work with a posi. But if you do that with a non-posi it doesn't work. It doesn't come out anywhere near right. What gives? Dave.
Melting Slicks
Joined: Jan 2004
Posts: 3,247
Likes: 210
From: Detroit MI
If I remember right I believe I am turning about 2700 rpms at 65mph. I had a formula for rpms to mph but I might not have written it down right. It was: Mph = tire dia x rpms / axle ratio. If I transpose that for axle ratio it would look something like this:
axle ratio = tire dia. x rpms / mph. but no matter what I plug into those values it doesn't come out right. So skip that idea.
I'm thinking it is a 3:08 now too. But here's another brain teaser. If you turn the wheel one revolution and count the turns of the drive shaft, that seems to work with a posi. But if you do that with a non-posi it doesn't work. It doesn't come out anywhere near right. What gives? Dave.
axle ratio = tire dia. x rpms / mph. but no matter what I plug into those values it doesn't come out right. So skip that idea.
I'm thinking it is a 3:08 now too. But here's another brain teaser. If you turn the wheel one revolution and count the turns of the drive shaft, that seems to work with a posi. But if you do that with a non-posi it doesn't work. It doesn't come out anywhere near right. What gives? Dave.
Also, if you spin the driveshaft (instead of turning the tire) and count the turns for one rev of the tire it should work no matter posi or not.
Instructor
Joined: May 2013
Posts: 158
Likes: 1
From: Ct
Yep, thanks. I did count the driveshaft turns to one rev of the wheel. For years I thought this thing had a 3:36 gear. But in counting it today I got to 3 turns of the shaft and then when the wheel got back to its one turn starting point the shaft did not make it to even a 1/4 turn more. I thought, hmm, that ain't .36. Did it again and came up with the same thing, just a smidgen past 3 turns of the driveshaft. That's when I started looking for numbers. Started thinking it must be a 3:08. Just looking for confirmation now. Thanks again.
I'm going to put a new engine in this winter and needed to know what I was working with. Dave.
I'm going to put a new engine in this winter and needed to know what I was working with. Dave.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Race Director
Joined: Dec 2004
Posts: 10,332
Likes: 63
From: PARADISE
St. Jude Donor '09-'11-'12-'13-'14
What rear tires are you running?
Race Director
Joined: Dec 2004
Posts: 10,332
Likes: 63
From: PARADISE
St. Jude Donor '09-'11-'12-'13-'14
Looks like the calc was asleep(or I was). Guess I'm getting too old to do stuff after dark except sleep.
At 26.3 diameter, he should be running 2575 rpm at 60 mph with a 3.36 rear gear, correct?
Last edited by claysmoker; Dec 12, 2014 at 02:50 PM.
Le Mans Master
Joined: Oct 2007
Posts: 7,454
Likes: 977
From: RI, Now Franklin/Nashville TN
As a reference a 255/60/15-27 inch diameter tire-which was a stock tire size beginning in the mid 70's with 3.70 gears and a 4 speed-1:1- will turn 2,860 RPM at 60 MPH-this information is from Road and Track-1978.
No guessing: Go to tirerack.com, put in the tire size you want, click on Specs, and this site will give you the tires overall diameter, as listed by the the tire manufacturer. I chose 255/50/17 ZR's when I changed rims since I wanted the exact same diameter of the 17's as when I used the 255/60/15-27.1-since I had 3.70 gears out back. Fronts are 255/45/17 ZR's-26.1 overall diameter.
No guessing: Go to tirerack.com, put in the tire size you want, click on Specs, and this site will give you the tires overall diameter, as listed by the the tire manufacturer. I chose 255/50/17 ZR's when I changed rims since I wanted the exact same diameter of the 17's as when I used the 255/60/15-27.1-since I had 3.70 gears out back. Fronts are 255/45/17 ZR's-26.1 overall diameter.
Last edited by jb78L-82; Dec 13, 2014 at 06:30 AM.