axle ratio

[marvxyz1]

Does any one know how to determine the axle ratio? I have a 1980 GMC and I've determined that my drive shaft turns about 1.33 turns when the wheel is rotated one turn. What would be my axle ratio? I've checked a speedometer site, and if my ratio is 1.33, then they don't have any listing for it. Anyone have any ideas?? Thanks for any possible help.,.. Marvxyz

[Prerunner1982]

Its most likely a 3.08 or 2.73...

[msgross]

well, if only one wheel turns then 1.33 = 2.66 for one drive shaft revolution... sounds like 2.73 If you are ready to change the diff lube then pull the cover and read the numbers on the ring gear. Divide the numbers (i.e. 41:11 = 41 divided by 11 = 3.72727272 or 3.73 gears)

[marvxyz1]

Sounds like I'll have to pull the diff. cover to get the ratio. What I''m trying to do is get a speedo gear which work with my truck. I'm ashamed to say, but I lost my original speedo gear in the transmission. Which leads to another question, how do you prevent the gear from slippijng into the transmission??

[marvxyz1]

One other questuion , I turned only one wheel, do I need to turn both wheels to get the right number?

[Duro5341]

Becareful, I could have sworn I had 2.73 in my truck, But I pulled the diff cover to do a service, and I have 2.86's....first time I've seen that gear

Duro

[msgross]

nope, open carrier means you have to spin the wheel twice... limited slip means that you have to spine both wheels once....

If you want to be sure then just pull the cover....

[PhilaTruck]

Yep, I agree, most likely a 2.73-

Here's the procedure for both open and posi rearends.

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.

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".

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.

"Posi" differentials: Secure the car from rolling forwards or backwards and place the car in neutral. Raise the rear end of the car so both rear tires are off the ground. 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 as you now need to be in neutral.

This procedure relies on the fact that your left and right wheels (thanks to the posi) will rotate the exact same number of turns as each other. Place a mark on the tire at bottom-dead-center. Crawl under the car and place a mark 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".

Have a friend turn the left wheel for 10 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, 50) and divide by 50. You're done! You can put the car back on the ground now.

Copied from: http://www-unix.oit.umass.edu/~tcroy...fferential.htm

But I'd just pull the diff cover if you don't know or remember the last time it's been changed.

Good luck-

-Todd

[jimfulco]

It's hard to watch a tire mark and a driveshaft mark at the same time. Just position the valve stem at 12:00 and then grab the valve stem and give the wheel two full turns by feel.