how do you determine rear gear ratio?
- Thread starter dave68coupe
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dave68coupe said:
If the rear is the original, the door ID will give you that info.
If not, here is what I would do:
Jack the car up.
Mark the tire with a chalk line at the top.
Mark the drive shaft with a chalk mark on the side.
Turn the tire 10 times while someone else is counting the number of time the driveshaft makes a full revolution.
Devide the number of revolutions by 10, this is your ratio.
Thanks for the replies guys
Found a goodwebsite and here is the answer
Determining which kind of differential you have
1. The vast majority of cars have an open differential as it is cheaper to manufacture, has no clutches to wear out, and is generally better-behaved. But posi units are out there, sold under titles of "Traction-lok", "Sure Grip", "Limited Slip" and the like. The concept is to place a clutch pack in-between the left and right end-gears, trying to force both wheels to turn the same speed at all times. Of course, to turn a corner, the wheel speeds must vary - thus the clutch-pack friction between the two end gears cannot be set extremely high or you'll suffer from hard steering (the car will want to continue in a straight line) and accelerated tire wear. OK, I've said enough. You get the picture.
2. Secure the car from rolling forwards or backwards and place the car in Park (if you have an automatic) or first gear (if you have a manual trans). Raise the rear end of the car so both rear tires are off the ground. Place jackstands under the car to safely support it.
3. Try to rotate the left wheel. You have "locked" the driveshaft from turning by placing the car in Park (or in gear), thus posi or no posi, the other wheel MUST turn the opposite direction at the same speed. If you have an open differential, this will take minimal effort. The wheels will rotate in opposite directions at the same speed. If you have a limited-slip differential though, the clutches don't like to let this happen so they'll fight you. A lot. A tight posi might take over 100 lb-ft to get the wheel to turn. By this point you'll know which procedure below you should use.
4. Open differentials: Secure the car from rolling forwards or backwards and place the car in neutral. Raise ONE SIDE of the car off the ground (left in this example) and place jackstands under the car to safely support it. If you just did steps 1-3 as a test, note that this is different from step 2, requiring you to change your setup.
5. Place a mark on the tire at bottom-dead-center. Crawl under the car and place a mark on the driveshaft. You'll be counting how many times the driveshaft turns in this test, so place the mark at a convenient place that you can call "zero".
6. Have a friend turn the left wheel for 20 rotations. Have them count to themselves while you count driveshaft rotations. When they are completed, divide the number of driveshaft revolutions by 10. For instance, 30 turns is a 3.00 differential ratio. 37 and a third turns is a 3.73 ratio. If you know the factory-available ratios, this will put you close enough to quickly determine which ratio you have. If you don't know the factory options, then do a larger number of wheel rotations (say, 100) and divide by 50 for a little more accuracy on that last decimal place. You're done! You can put the car back on the ground now.
Found a goodwebsite and here is the answer
Determining which kind of differential you have
1. The vast majority of cars have an open differential as it is cheaper to manufacture, has no clutches to wear out, and is generally better-behaved. But posi units are out there, sold under titles of "Traction-lok", "Sure Grip", "Limited Slip" and the like. The concept is to place a clutch pack in-between the left and right end-gears, trying to force both wheels to turn the same speed at all times. Of course, to turn a corner, the wheel speeds must vary - thus the clutch-pack friction between the two end gears cannot be set extremely high or you'll suffer from hard steering (the car will want to continue in a straight line) and accelerated tire wear. OK, I've said enough. You get the picture.
2. Secure the car from rolling forwards or backwards and place the car in Park (if you have an automatic) or first gear (if you have a manual trans). Raise the rear end of the car so both rear tires are off the ground. Place jackstands under the car to safely support it.
3. Try to rotate the left wheel. You have "locked" the driveshaft from turning by placing the car in Park (or in gear), thus posi or no posi, the other wheel MUST turn the opposite direction at the same speed. If you have an open differential, this will take minimal effort. The wheels will rotate in opposite directions at the same speed. If you have a limited-slip differential though, the clutches don't like to let this happen so they'll fight you. A lot. A tight posi might take over 100 lb-ft to get the wheel to turn. By this point you'll know which procedure below you should use.
4. Open differentials: Secure the car from rolling forwards or backwards and place the car in neutral. Raise ONE SIDE of the car off the ground (left in this example) and place jackstands under the car to safely support it. If you just did steps 1-3 as a test, note that this is different from step 2, requiring you to change your setup.
5. Place a mark on the tire at bottom-dead-center. Crawl under the car and place a mark on the driveshaft. You'll be counting how many times the driveshaft turns in this test, so place the mark at a convenient place that you can call "zero".
6. Have a friend turn the left wheel for 20 rotations. Have them count to themselves while you count driveshaft rotations. When they are completed, divide the number of driveshaft revolutions by 10. For instance, 30 turns is a 3.00 differential ratio. 37 and a third turns is a 3.73 ratio. If you know the factory-available ratios, this will put you close enough to quickly determine which ratio you have. If you don't know the factory options, then do a larger number of wheel rotations (say, 100) and divide by 50 for a little more accuracy on that last decimal place. You're done! You can put the car back on the ground now.
D.Hearne said:
yes it does man....
jack one wheel up and leave one on the ground for an open diff....
spin the wheel one full revolution and note how many times the driveshaft spins, with this method your driveshaft will either spin 1/2 or double the amount compared to a limited slip, locker, spool etc.... youll know once you do it......i think the drivshaft turns half as much with this method...
it works trust me iv done it....
-gbm-
D.Hearne said:Maybe I just like to practice my algebra skills more than you do GBM
I been chasing the ladies man, add me to your msn ill give ya a few stories
Im gonna try and check in a little more here, i see we have a new format 
but seriously my method works well, and im almost 97% sure the drivshaft spins half as much, so spin the tire once and count the drivshaft revolutions and double it.... so if the gears are 350:1 the drivshaft will spin 1.75 with a open diff with one tire on the ground.....
i aswell tried to figure it out with both wheels up in the air but i couldnt get an accurate reading.......
if you can do the math spin the tire more times as that will get it even more accurate.......
-gbm-
I think that the few times I tried that with the 67 coupe I had a couple years ago, that's what I kept coming up with, half the revolutions than it should have. It had a 2.79 8" rear and the driveshaft would spin about 1 1/2 times for one tire rev. As far as adding you to my msn,I'm not sure on how to do that 
( after three years or so, I'm still learnin this stuff) Or even If I should the wife may not like me reading those stories 
( after three years or so, I'm still learnin this stuff) Or even If I should the wife may not like me reading those stories Similar threads
