Inliners International Forums General Discussion Drive Train Rear ratio?

Can anyone help with the stock rear ratio for a 1970 GMC 1/2 ton 115wb with a 250 and powerglide transmission? I cant find it in the owners or shop manual or the glove box decal. Its also a 6 lug.

Thanks Mac

I think it's jack up one rear tire. Mark the drive shaft and turn the tire 2 times around and count the number of times the drive rotated. A little past 3 will be a 3.08. A little past 3 1/2 is 3.55 etc...Good Luck

If it has not been messed with there may be a tag held on by a cover bolt. Some were stamped on the right, I think, axle housing. Last resort, it's stamped on the ring gear.

I'm going to crawl underneath the truck tomorrow and check for a tag. If not I'll try the drive shaft rotation trick. I'll be pulling the engine tomorrow starting the rebuild.

Mac

If you are gonna change the rear axle oil anyway... look at the ring gear. You should see numbers stamped into the ring gear... like 15.41 Then just Just divide big number by little number to get the axle ratio. Example 41 divided by 15 equals 2.73

Thanks to the guy who runs NastyZ28 website. See page for details http://www.nastyz28.com/camaro/zsusp.html#rearaxle

If it's a non posi,jack up one rear tire off the ground,rotate the tire one full turn while counting how many turns the driveshaft make.

Lets say the driveshaft turns a little over 1.5 turns,you would multply it by 2 which will be approx 308's

With a posi unit,,,jack up both tires off the ground,rotate tires one full turn & count how many turns the driveshaft,it will be the exact ratio,so if your driveshaft turns,say a little over 4 turns,most likely it will be 410's

Hope this makes sense.


MBHD

Good possibilty it is a 3.73 ratio. A lot of the trucks in that era had this ratio in it. My '65 had it as well.

Its a 3:73! Thanks

Just a suggestion....instead of turning the rear wheel over one time and trying to eyeball whether you've got a 3.08, 3.23, 3.42 or a 3.73.....turn the wheel 10 revolutions while counting the driveshaft rotations. By doing it this way you not only determine the first number of the R&P ratio, but also the second. As an example, if 10 turns of the wheel give you 17 revolutions of the driveshaft. Multiply that by 2 and divide by 10 and you have 3.4 which would determine that you have narrowed down the ratio to the 3.4's. If you're trying to nail the R&P ratio it helps to have a more accurate count.

Dave