There is something I don't understand here. In trying to figure out whether I have a 3.50 or a 3.55 ratio rear (probably a 3.55 since it is an 8" Ford rear, but the shop guy insisted it is a 3.50) my son and I tried the chalk mark trick. We jacked up the right rear wheel and the left rear stayed fixed. We counted 35.5 shaft rotations for 20 rotations of the right rear tire and 71 rotations of the shaft for 40 rotations of the right rear tire. Thus the procedure given by Tech1 gives us 1.775 which is unlikely. I think the problem is that you have to know if you have a positraction rear or an open-differential. I agree that the method Tech1 gave is correct for a positraction rear, but in my case I think the factor of 2 is caused by the differential spider gears?? With that factor of 2 included we get 3.55 and give the warning that one needs to know if you have a positraction rear or and open differential.

Don Shillady
Retired Scientist/teen rodder