The Stovebolt Site Forums The Shop Area Engine Shop 1970 C-10 Rear End Gears ?

I'm having a real problem finding out what gears my rear end has. All I have are the casting numbers-NF 3896838. I found that is from a 1971 C10. Here is my confusion. I jacked it up and marked the drive shaft then turned the wheel and it went 2-1/4 turns for 1 shaft turn. That don't make sense! It has a 250 L6 and a SM465 4 speed (not orginal). There is no other ID on it.
Thanks from a new guy.
Jim

Welcome to the job dodad70!

Here's the most accurate way I know to do this. Do just what you did only turn the wheel 20 times while counting the driveshaft revolutions, then divide your driveshaft number by 10
(this is for a non-posi rear, only jack up one side)
For a posi rear, spin the wheel 10 times (both sides jacked up) and divide by 10.
Sounds complicated but it really isn't. I use a sheet of paper divided into 2 columns and just make "tick" marks until I get to 20 turns.

Also , if you look on the inside of your axle on the passenger side,next to the pumpkin, you will see 2 or 3 letters followed by some numbers stamped in the axle tube, they are hard to see,get your wire brush out! then go to this site http://www.americanclassic.com/paint_test.asp and find out. Mark

If you're only jacking up one wheel, the action of the spider gears doubles wheel speed. You'll have to make two turns of the wheel to get an accurate estimate of the ratio. For instance, if the driveshaft makes just over 4 turns to get the wheel to turn twice, You've got a 4.10:1 ratio gearset.

A more accurate way is to make a chalk mark on the driveshaft, and roll the truck straight forward one full turn of the rear wheels, counting driveshaft turns as you go. Since both rear wheels are on the ground, the speed-doubling function of the spider gears doesn't become a factor. That's why we never exceeded 30 MPH when doing on-car wheel balancing. The actual wheel speed was 60.
Jerry

Another way and even more accurate, is take off the cover & drain the oil, look at the outside edge of the ring gear. You should see the number of teeth of both the ring gear and the pinion gear stamped on the edge of the ring gear.
Divide the 2 numbers and there is your gear ratio. You may have to round off the 3rd didgit.
For example on a 4.10 ring & pinion you would see a 41-10 stamped on the ring gear, meaning 41 teeth on the ring, & 10 on the pinion. Divide the two & there's your ratio. Or you can count the teeth on both gears and divide them that way.
Try it.

If it's an unmolested truck, there's an axle code printed on a build sheet inside the glove box. List that code and someone will decipher it for you. (My guess is you have a 3.08:1)
Scott