O.K in the past Audiovox Cruise control (now Rostra) sold some magnet stuff to stick on the drive shaft and a sensor... i remember that time.

Yup, hall effect sensor is the way to go. Now you can begin by using a 16 bit internal counter and read the results after 1 mile. You should be able to do something with it.

Read datasheet in the TIMER section. Once done, do some test.

In the Code Example section, you can find a Frequency Counter thread. It should gives you some start point.

Good luck!