So if the tire is 781 revs per mile
And the rear is 3.91
The gear on the transmission is 13 teeth
And the speedo gear is 40
This should get a cable RPM of....
781×3.91×13÷40=992.5 RPM
So if the tire is 781 revs per mile
And if the rear is 3.73
The gear on the transmission is 13 teeth
And the speedo gear is 38
This should get a cable RPM of....
781×3.73×13÷38=996.6 RPM
You have tried both those gears and you are still off.
I think we need to nail down your rear gear ratio with some accuracy
But it's starting to look like your speedo is in need of calibration
(These numbers should get you to about 1000 RPM speedo shaft speed and as such 60 MPH (1 mile per minute))
