physics, best toy car...

Weight - Depends on your ramp distance and angle, you want your toy car to launched at its highest speed before the ramp runs off. You can adjust your weight to meet this, typically the heavier the better, since inertia is resistance to movement. Aerodynamic - Get your car in a nice and aerodynamic shape, like a normal car will do. Wheels - Don't get cheap wheels, you don't want them to wobble while moving, since alignments and tolerance is important to keep the car on course and absorbing the uneven surface, as well as minimize friction as much as possible on your set of wheels to the shaft that is holding your wheels. Lubricating them would be a great idea to eliminate these frictions. Hope these helps, good luck and happy mylotting