because everything is really light weight and only limited amount of input energy is allowed, drag will play a big part in slowing it down. Make sure you minimise that, otherwise it will stop real quick. As you already know ( I hope) what you are doing is converting potential energy to kinetic energy. Make sure you set it up so that the conversion rate is very slow, you want the weight to drop slowly and propel the car as it decends. Having one quick burst of energy will not be good in my opinion. The reason is the same - drag. High speed means lots of drag, keep the speed low and this will keep the drag low. Same applies to friction in the moving components. It is not a speed race, it is a distance race.
That is my opinion anyway. Oh and you really need to do something about your spelling
Good luck
