Drive up behind a car and go into "bullet time". Then, aim at the license plate and fire. Most of the time it will take a single shot. For cargo trucks, aim under the passenger side and slightly to the back and fire. This makes stopping street racers and other people easy.
To explode a car in one hit, go into aim mode and wait until your aiming crosshair appears. Then, shoot when it turns red. If it does not turn red, then just shoot around the license plate until the car explodes.
