Although in real life, body kits were originally designed for aero-dynamics (to reduce wind resistance, thus making your car move faster), this is not true in the game. The body kits weigh down the car more, thus reducing your top speed. This can be as little as 10 mph to as much as a 25 mph loss, which can obviously be the difference between walking away with someone's ride or losing yours. Do not put body kits on your cars. They do not add to respect, will slow you down, and makes your car cost more to repair.