Hello BrianD,
Setup a timer to overflow at a rate you can use to determine seconds easily and track those overflows in a count variable. have switch, reset the timer and counter and send data to your display by using an interrupt routine. Assuming you are talking about a Slow RPM.
So let's say timer overflows every 10 ms, then 100 overflows = 1 second, so if interrupt occurs after 250 overflows you know it went 2.5 seconds, so you have data to cypher out the rate of travel. Clear as mud?
Probably 10ms is a bit far fetched but the numbers were easy.