It’s a pretty simple bit of figuring, as figuring goes. Figure out what the circumference of the tire is, and that’s your distance per round. Convert that from inches or feet, to miles. Now, figure what the RPM of the wheel has to be to get your target MPH. Then figure the reduction from the RPM of the motor to get to the RPM of the wheel.
Give up? Go here: CompGoCarts has a great interactive calculator.
For the record, I did the math and my motor, with my wheel and tire, came out to almost exactly what I had on there from the stock configuration- a 13 tooth front and a 45 tooth rear. It calculated out to a 65mph top speed at the RPM maximum of my motor- 3000, with load. The actual top speed of my bike? About 63mph- not bad. It always is a pleasant surprise when math actually works.
While we’re on the subject of calculators, if you want to crunch some numbers on theoretical specs, there’s nothing better than the Lennon Rogers EV calculator.