Move the sensor and magnet to the crank.

The first instinct is to have 1 MPH = 1 RPM. However, typically you will be spinning up at around 100 RPM, and most bike comps can't really deal with a speed of 100 MPH (or faster), so since it probably reads in tenths, I'd shoot for the RPM reading to be in tenths (IE 10.0 MPH = 100 RPM)

The math is ridiculously simple.

You want 10 RPM to be equal to 1.0 MPH on the comp. Since 1 mile = 5280 feet, and an hour = 60 minutes, in 60 minutes your pedal (which it thinks is a wheel) turning 10 times a minute will turn 600 times. So you want to set up a circumference where 600 times = 5280 feet. That means a circumference of 5280/600 = 8.8 feet. Since I think all comps want to be set up in millimeters, convert that. 8.8 feet = 105.6 inches = 2682.24 millimeters.

Set the comp to a wheel circumference of 2682 mm.

EDIT: sorry about duplicating your comments on 100 MPH, tenths readout, etc. I just typed what I normally do for this question.