This only works on "posi" LOL (limited slip) rear axles. If you jack up one wheel and it turns you do NOT have limited slip. No do the the above, only count shaft turns and turn the tire TWO full turns.
Also, I've bought several GPS units from C/L and local thrift stores. They make a cheap easy and ACCURATE speedo
I agree^^the tach could be off as well. You can actually check a tach by downloading some musical notes (tones) running your sound output through a low to high impedance transformer (to generate higher voltage) and using that to feed the tach trigger wire. You have to calculate the tone frequency to V8 trigger frequency
I posted a thread on this years ago, never find it now!!!
