I have a caution about the speed sensor. If the sensor you use has more than 1 pulse/rev. then the angular differance between the successive pulses might not be the same over 1 revolution of the sensor. This means that you could get different elapsed times between 2 pulses depending upon where you are in the sensor's "circle" for the same speed. The solution for this type of error is to take the average of all the pulses in the sensor's "circle". Another solution is to obtain a figure for the max. angular differance between 2 successive pulses & use this to figure the max. error for this type of error.