Ok, here's my thoughts, after reviewing all of your (mostly) thoughtful replies:

First of all, I don't want to tie up my processor while a count takes place.
Second, here's what I came up with. Please poke the required number of holes in it.

My tire circumference is 78.4" and I have 5 rotor bolts. so between any two pulses, the tire travels 15.68". If I measure the time it takes to travel 15.68" and multiply it by 4040.82, I should have the mph. I divided 15.68 into 63360, which is the number of inches in a mile.

Does that make sense?