speed = d/t

speed is measured in kph

t = time

d = distance

so for 30 kph

t = 1 hour

d = 30 km

distance is calculated by number of wheel rotations WR * wheel circumference C

assuming the loaded wheel diameter is exactly 26" (which it's not but, for the sake of making the math easy we will assume it is) we can calculate the circumference of the wheel.

circumference = pi * the diameter = 3.14 * 26 = 81.64 inches

30km = 1,181,102.36 inches = 18.6411358 miles

wheel circumference = 81.64 inches

# of rotations = 1,181,102.36 / 81.64 = 14467.20

d = WR * c = 14467.20 * 81.64 = 30km

1 Kilometer = 0.621371192 miles

So the magic number MN is .621371193

If we multiply the wheel circumference by 0.621371193 we get

81.64 * 0.621371193 = 50.72874419652 as the adjusted wheel circumference

to convert inches to kilometers multiply by .0000254

put that back in the equation to check our work

Number of rotations * Wheel circumference * Magic number * Conversion factor = MPH

30Kph = 14467.2 * 81.64 * 0.621371193 * .0000254 = 18.6411334

so if you want the speed measured in Kph to display Mph so that the number is correct you will need to multiply either the wheel circumference or the loaded wheel diameter by .621371193

I just checked the date of the original post and it's a bit old. Oh well.