Darrel Yes I am viewing drunk so this may be wrong

120 mph = .000125uS per pulse
60 mph = .000250uS per pulse
10 mph = .001500uS per pulse
6 mph = .002500uS per pulse

(needs to be rechecked) this is with 4000 pulses per mile
now if you use 20 MHz you have .000002uS resolution.