The Stovebolt Site Forums The Shop Area Driveline Rear axle gear ratio

I have a 49 GMC 1/2 ton that I bought as a stalled project. It has a rear end in it that has a casting #14065251. I was lead to believe this is from a early to med 80's Blazer. When I tried to confirm the gear ratio ( jacked up the rear end, chucked one wheel to keep it from spinning I got just over 2 revs on the drive shaft for 1 rev on the tire opposite tire. Did GM make a 2.08 rear gear set? Or am I doing this wrong?

Tom

O.E. rear end would have a round cover & the 3rd member bolts into the housing.
2.08 would be about twice the speed of o.e.
Remove the rear cover, should be markings on ring gear, smaller # is pinion teeth, larger number is ring gear teeth. If no marking found count teeth on ring & pinion, ring gear teeth by drive pinion teeth will give you the ratio.

There should be a number punched in the axle tube that identifies gear ratio, etc.

I believe the way you are checking it, you need to double it (because you are only spinning one wheel). That would be closer to a 4.11 gear set. It would be best as suggested to pop the rear cover off and look on the ring gear.

There is ONLY one way to be sure what is in there and that is what B59 and 69Cuda said. Cost: One gasket and some gear lube.

It’s not that hard or difficult :

http://www.how-to-build-hotrods.com/gear-ratio.html


Need a video..........there are dozens on youtube




As far as the OP. GM did make a 3.08 open rear differential.

Here is what that link says:

"If it takes about 2 and three-quarters turns, it's around a 2.73 gear ratio. If it's a little more than 3, you have 3.08s. If it takes about 3 and a quarter turns of the driveshaft, you have 3.23 or 3.25 gears. Very close to 3 and three-quarter turns, 3.70 or 3.73 gears. This method won't work very well on a posi that's worn-out and won't spin both tires equally"

Agree, there are ways to be close. With a friend, some tape, a jack, some jack stands, mark your driveshaft and mark your tires and you are all set to estimate. That can be suffice for many.

Not sure if the "we don't guess, we test" adage, by Hot Rod Jerry, would be satisfied.

If the link does not work: Google..... “checking rear differential ratio”........it will yield 50 links with directions, videos, diagrams.

Thanks to everyone for the info. I did try again with one wheel blocked and rotated the other 10 times and counted the drive shaft turns. The driveshaft turned 18.7 times. Take that times 2 and divide by 10 and you get 3.74. The gears are 3.73s

I think you should still try to find the number stamped on the axle tube as you seem vague as to what the rear end came out of. The number should tell you year and vehicle. I found this info useful when I bought parts such as brake shoes. In may case, I tell the parts guy that the parts are for a 1978 Camaro with posi. It wouldn't be useful to tell him that I want parts for a 54 pickup as they won't fit!

I bought this truck as a stalled project in New Mexico. It was sitting completely disassembled in a garage since 1998. As I understand,they (a father and son project) bought a early 80's Blazer for the drive train and a few other parts , however being 20 years ago the memories are vague. I have the casting number from the tube in my notes for future reference.