potential energy of the car is Pe=m*h*g where m is mass of the car,h is height from top to bottom of the stand, g is acceleration due to gravity. Kinetic energy is Ke=1/2 *M*V*V where v is velocity. at top of the ramp Pe is max, Ke is 0, at the bottom of the ramp Ke is max, Pe is 0. Drag energy we can make De=Fd*D where d is distance, Fd is drag force. SO Pe turns into Ke and that turns into De. the limiting factor is Fd, which is force of drag, comes from wheel drag, air drag, etc... Just by looking at this, the car must be made heaviest so it will have maximum Pe which will yeld maximum Ke. As long as Fd is small, then D will be largest.