Thanks for your reply.
I am planning to make a stand alone system for speed registration for chairs driving over the floor sensors. and not a speedometer for the chair.

I have made a board with a pic prossesor and LCD. i know how to programme this.

What i dont know (a lots of stuff) is how to set up a timer inside the pic to give me the time it takes from the start sensor are trigged to the stop sensor stops the counter.
i also need help to do the mathematics so i can convert the time/m to Km/h.

Dag