Rear Gear Ratio Decipher?
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
Rear Gear Ratio Decipher?
While I had the car jacked up doing calipers last night I decided to try and figure out my gear ratio is in my 78......the drive shaft turns one full turn to the tires 3/4 turn...not good at math concepts....approximately what ratio do I have?
3.34??
3.34??
Race Director
Joined: Feb 1999
Posts: 16,978
Likes: 7
From: In Dreams There Is Truth Ohio
use chalk, mark the differential and the yoke, also mark the tire at either 12 or 6 o'clock then rotate the tire and count how many times the chalk mark passed the mark on the diff for one rotation of the tire. Don't try to eyball the rotations, use chalk.
Racer
Joined: May 2005
Posts: 470
Likes: 1
From: Amelia Ohio
Mark the wheel and ujoint. You should get a minimum 3 turns of the joint to 1 turn of the wheel. Less than 3 and it is a good highway cruiser and lousy at the light. I think most were 3.55 or 3.73.Chris
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
OK guys, Put the tranny in neutral, crawled under the Vette again. Chalked the drive shaft (not the half shaft) and put a chalk mark on the tire. The drive shaft rotates 1 turn to the tires 3/4 turn. Doesn't make since to me ..what the heck kind of ratio would that be??? I know it doesn't sound right..............
Race Director
Joined: Dec 1999
Posts: 19,610
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From: Forked River NJ
That is impossible unless your rear end is really bad. Did you watch the other tire also. Are they both spinning the same way. They should be both spinning the same way with a posi rear end. Sounds like something might be wrong. My drive shaft spins 3 3/4 turns for every wheel rev. Hence a 3.70 rear.
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Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
What's the highest ring and pinion combination that could go in a 78 carrier? Right now with those rotations, I'm looking at an impossible 1.33 ???? 
Just don't undestand what's going on.........
Just don't undestand what's going on.........
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,501
From: Little Rock AR
Originally Posted by Hvymtlc5
OK guys, Put the tranny in neutral, crawled under the Vette again. Chalked the drive shaft (not the half shaft) and put a chalk mark on the tire. The drive shaft rotates 1 turn to the tires 3/4 turn. Doesn't make since to me ..what the heck kind of ratio would that be??? I know it doesn't sound right..............
Both tires need to be off the ground and rotating together. You will see something like 3 full driveshaft rotations for 1 full wheels rotation.
You probably have about a 3.08:1 ratio (~3:1 ratio) in that differential so 3 driveshaft rotations will give you 1 wheel rotation.
-Mark.
Race Director
Joined: Feb 1999
Posts: 16,978
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From: In Dreams There Is Truth Ohio
Tell you what you do....make another mark on the diff and line the driveshaft mark with the diff...then place a mark on the inside tire at 6 o'clock and slowly rotate the tire and count the number of times the drive shaft mark passes the diff and stop when the tire is on the 6 o'clock position
Whatever you are doing, you are not doing it properly
Whatever you are doing, you are not doing it properly
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
I know what I'm seeing - baffles me too - that's why I'm posting Is there a possibility that someone had a very high ratio rear end installed back in the 70's?? With those rotations (2 driveshaft to 1 1/4 tire rotation) it would be extremely high.
For sanity, I will check it once more in the a.m.
For sanity, I will check it once more in the a.m.
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,501
From: Little Rock AR
One more thing to look at...Make sure that both wheels are rotating the same direction. This will happen if you have a limited slip differential. If one wheel rotates forward and one rotates backwards you do not have a limited slip differential. That could be a problem with this procedure.
-Mark.
-Mark.
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
Didn't all Vettes have a limited slip? Anyway, both wheels turn the same direction.. and the 1 1/4 wheel rotation to two driveshaft rotations still stands...... 
Any other ideas?
Any other ideas?