Ok if you have the 2.59 gears it should run something like this.
In Drive, 45mph = 1520rpm, 65mph = 2200rpm
In OverDrive, 65mph = 1540rpm, 84mph=2000rpm
That is an approximation.
If you're 200-400rpm higher in OD then it's probably the 3.07.
You could just jack up the rear end, mark the drive shaft, and then turn your rear wheels one complete turn. All you have to do is count how many times the shaft spins. (A string taped, and marked for each rotation helps with that fractional part.)