Calculate ratio from speed @ 1000rpm

[viceroy]

Is there a calculation by which I can convert mph @ 1000rpm in a gear to a gear ratio?

ie from 5mph @ 1000rpm in 1st to 3.06:1 (just as an example)

 

[BenR]

you would need to find out your diff ratio first....but yes you can.

but from speeds, it wont be very accurate, you will need to count the number of rotations of the diff in each gear.