It is probably a constant that is part of the formula. Many of them have them as part of their calculation process, and is a shortcut to keep from having to use a bigger formula.
The hidden constant in the calculation is PI - the coefficient 336 comes from the scalars in the formula: miles per hour, revolutions per minute and tire diameter in inches. The number can be backed into with a thought exercise.
Imagine a 1" diameter tire.
There are 5,280 feet per mile or 63,360 inches per mile.
Circumference = PI * diameter
So for a 1" tire we have 3.141592654 inches in circumference (or inches traveled per revolution)
Arithmetic leads us to:
Revs per mile 20,168 (60,360 / Circumference)
Revs per min per mph 336.1352398 (20,168 / 60)
Or 336 inches per minute per mph.
The value can be also derived in the following way.
There are 60 minutes in an hour and 63,360 inches in a mile
60/63,360 = 1/1056th mph.
1056/PI = 336.1352398 inches per minute per mph.
but I find the thought exercise easier to get my head around.
