Think about how to get the maximum amount of energy out of the falling mass. Then think about what would allow your vehicle to go as fast as possible on the smallest amount of energy. Then add in a method to stop the vehicle at a predeterminds distance. Keep in mind that skidding is unpredictable so any system that locks the wheels is likely to be unpredicable. Also keep in mind that you need to find a balance between speed and accurate stopping distance. The faster you go the harder it will be to get an accurate stopping distance.
The are many places you can go for ideas, just do a search of Science Olympiad Scrambler.