The Stovebolt Site Forums The Shop Area Driveline axle ratio

Hey! I am trying to figure out the ratio for my rear axle. I jacked up the truck and turned the wheel over by hand. The driveshaft only turns 2 complete turns for 1 complete turn of the wheel. Can anyone tell me what ratio this would give me?....2:1? It still revs. really high at 45mph. This is a 1955 Chevy series 2, 3spd, 235. The rear axle was changed at some point in time and I don't know what it is out of. Thanks for all your help.

if you only moved one wheel and left the other on the ground, then your ratio is off by a factor of 2x.. so you are looking at a ratio in the neighborhood of 4:1. the factory ratio for your truck is 3.9:1, so that makes sense.

You can get more precision in your test by turning the wheel 10 or 20 turns. I bet if you turn the rear wheel 20 turns, you'll find the driveshaft turns 39 turns.
2 x 39 / 20 = 3.9

regards,
Leon

THANKS GUYS. my next question is...What rear ends are a direct swap?...pickup, car? OR...Where do I find a gear set to it with, like 3.53, 3.23, etc?
Thanks again!

you might try tech tips...
also just do a google search,tons of stuff out there on these swaps

a lot depends on how you use your truck, city driving, pulling stuff, mountain driving, etc..

you can buy a 3:38 set from a variety of suppliers - here's one:

3.38 gear set

Reviving this thread. Jacked up my differential so both wheels were off the ground. Shaft spins if I turn the PS wheel but not the DS wheel.

1 full spin of the wheel will get 1.8 turns of the DS. I also counted 20 rotations of the DS and that took 10.75 turns of the wheel.

I think that gives me a 3.73 rear. anyone able to confirm my calculations?

How many lug nuts do you have? It could be a passenger car rear. If you have that kind of ratio, you would not be over revving at 45 mph. It might be noisy, but thats how they are.

Oh, sorry!

I have a 1965 C10 with 6 lugs and a 12 bolt rear diff cover. I am pretty certain it is the original rear.

Can anyone explain why the drive shaft does not spin when I rotate the DS wheel? It spins just fine when I turn the PS rear wheel. Also, the shaft will spin if I rotate the DS wheel in REVERSE. I guess it's a sign that I don't have a posi rear,right?

You've got three moving parts, the driveshaft, and both rear axles. Whichever one has the most drag will sit still, and the other two will move. If you just jack up one wheel and turn the other one, the spider gears between the axles will alter the speed of the other parts. One turn of the driveshaft will result in the wheel turning at twice its normal speed if both axles were free to turn. Remember this if you encounter wheelspin on one side in mud or snow- - -30 MPH on the speedometer will be 60 MPH at the spinning wheel.

The most accurate way I've found to estimate the rear axle ratio is to roll the vehicle straight forward exactly one turn of the rear wheels, and count the turns of the driveshaft. With both wheels on the ground, it's impossible to get an error induced by the differential gearing.
Jerry

Thanks HRL!
I can't roll it in the yard at the moment because I have a support under the frame while I work on restoring the control arms, springs, new shocks and brakes. It was just a good time to see the shaft turn while I spun the wheel. Anyway, my math comes out to 3.7209, so I'm pretty sure it's a 3.73 rear on my 65 C10.

I should be able to open the rear next week and clean and change oil. Probably will do new seals and bearings too. (As I say all that, my eyes roll back in my head because I have NEVER done it before and I know I'll be back on the forum with more questions! LOL!!)

Thanks for always being generous with help! I'll post some pics once I have some worthy of posting.

Follow up: Well, the method of turning the rear tire and watching the shaft spin really works pretty good. I counted my splines on the pinion and ring yesterday. Pinion = 11 and ring = 41, so yep, I have a 3.73 rear.