Hello i need to make a speed measuring device to check the top speed of electric wheelchairs.

I use two sensors on the floor to detect the chair. sending start and stop signals to my 16f877-20 pic.
the distanse between the sensors are 1000mm. i need the display to give me the correct speed presented in km/h on the display.
I need some tips in how to set up a counter in the pic and how to express the speed in km/h
The range is 0.0-20.0km/h.