Lego NXT Calculating force of gravity

I made a simple lego contraption to measure the force due to gravity.
The device consists of a lego retort stand with a switch at the top and a switch at the bottom.
We drop a lego tyre via a quick release mechanism. The tyre is connected from top to bottom by a piece of string that does nor impede the fall but guides the wheel down onto the switch at the bottom.
When we release the wheel at the top the timer starts and when the switch at the bottom is hit then the timer stops. Knowing the distance and the time then the NXT can calculate the force of gravity.