Hi all. I'm trying to determine the rear gear ratio on my OPEN GM 10-bolt rear. I just did the "spin the wheel and check the drive shaft revolutions" trick and determined it to be roughly 12.8 or 13.8 drive shaft (sorry, trying to count two different operations is challenging) to 10 wheel revolutions. I calculated it to be 12.8 x 2 /10 = 2.56 or 13.8 x2 /10 = 2.76 (2.73, maybe?). I don't know which is an actual 10-bolt GM rear ratio to know which one is the most likely. I was hoping (and was told) it was a posi rear but, nope!

Anyway, I would just like a list of the possibilities.

Thanks,
Greg
('68 Camaro RS/SS 427 4spd)

 

I believe the available ratios on an 8.2” 10-bolt in a Camaro were 2.56, 2.73, 3.08, 3.36 (possibly), and 3.55.

But it gets more complicated than that. Scroll way down the bottom of this reference:

1967-69 Camaro Drivetrain Decoding

Is that 8.2” 10-bolt actually surviving behind a 427?

 

It's always best to count driveshaft revolutions for one tire revolution. If you're under the car doing it yourself, put a piece of tape on each tire and on the driveshaft and turn the driveshaft to bring the tire tapes back to the original position. two tire rotations will increase the accuracy. If the tires don't match in a turn means the open differential is coming into play. Then it's best to remove the cover and look at the numerical designations on the ring gear.

 

Thanks for the info. That is exactly what I was looking for. I am looking into replacing the open 10-bolt with a posi unit. I may have just scored an 8.5" 10-bolt posi with three sets of gears.

Greg

 

I already determined the ratio with chalk, and found it to be an open rear to boot! In the market for 12-bolt posi or 10-bolt posi right now.

Thanks though,
Greg

 

Try for an 8.5” 10-bolt, if you cannot find a 12-bolt.