Adding to Alain's expert analysis of distance.

RATE * TIME = DISTANCE, so... If 1 rotation takes 1 second then:

78.4 inches per second * 60 seconds = 4704 inches per minute. Multiply by 60 minutes to find distance per hour = 282,240 inches per hour or (dividing by 63,360 inches in one mile) = 4.45 MPH.

-OR-

[INCHES PER SECOND] * 3600 / 63360 = MPH

The above works only if the inches in one second are known. So, we must also know how to calculate the [INCHES PER SECOND]... Take the example of .4 seconds per rotation:

To calculate the [INCHES PER SECOND] multiply the reciprocal (1/X) of rotation period by the wheel diameter (78.4) to find:
1 / .4 = 2.5 rotations per second. 2.5 rotations (2.5 * 78.4) = 196 inches = [INCHES PER SECOND]. Now, complete the formula above (11.136 MPH) . Not remarkably this is 2.5 times the 4.45 MPH figure calculated for 1 rotation per second - allowing for rounding.


DISCLAIMER: The accuracy of these figures is only as correct as the time base from which they are figured. IE: timer overflow vs atomic clock.