Heres the math for diameter in inches (26) and transforming the count N of 1 sec worth of spokes into MPH.

So,

Diameter = 26 and

T = 1.0

miles/hour = (N spokes / T seconds) * (1 revolution / 36 spokes) * (Diameter * Pi inch/ revolution) * (1 ft / 12 inch) * (1 mile / 5280 ft) *

(60 seconds / 1 minute) * (60 minutes / 1 hour)

MPH = N * 0.12892

You might want to update faster than 1 sec. Since you are counting spokes (as opposed to a magnetic, once per revolution sensor) you have that luxury.

Just multiply the above constant by the number of times per second you are updating. Ex. twice a second makes T = 0.5, effectively multiply the constant by 2 giving:

MPH = N * 0.2578

You should probably round MPH to the nearest tenth MPH with something like:

roundedMPH = (float)((int)(MPH * 10.0 + 0.5)) / 10.0;

Say N is 57 for one interval,

then MPH = 14.6946 and you display

rounded MPH = 14.7