How to read RPM data from car with micro-controller?
Measuring frequency might seem hard thing to do but probably isn't. Period and frequency are inversely proportional so getting one means you are free to calculate another.

Let's talk about examples. I will use AVR205:Frequency Measurement Made Easy 
with Atmel tinyAVR and Atmel megaAVR Application Note as a starting point. As you can see it is suggested to use timer and some source to start counting. Count how many peaks there during some time and recalculate. Be aware of tradeoffs - short amount of time makes system fast yet not as precise, also, using external clock would make measurements even better when introducing some small additional cost.