Hello,
Trying to determine what my current gear ratio is and I can't find any markings so I tried jacking up the rear end and counting how many times the driveshaft spins for 10 spins of the tire but the number I got, 14, doesn't make sense from what I've read because I was expecting between 25 and 30. It's a 1968 Cutlass S with an O type open diff. Here's a video and some pics. Am I doing something wrong to figure this out?

 

Since only one wheel is turning, you need to multiply by 2 to take into account the differential action.

14 x 2 = 28
28 / 10 = 2.8

So it is likely a 2.78:1 ratio rear.

 

Thank you very much for the information!