A car typically needs 10-20 horsepower to move at constant speed on a level surface. Where do you think most of the energy is dissipated under these conditions?

The energy dissipated by the car is due to the effects of friction and air resistance.

Explanation:

When moving at a constant speed at level surface, the car does not need to overcome it's weight as it acts downwards.

The only two things that are stopping the car from moving any faster will thus be air resistance and friction.

The friction is present between the car tires and the road.

The air resistance acts on the body of the car, as air tries to stop the motion of the car.

Thus, friction and air resistance dissipate most of the energy of the car.