Certain cars that have the option to add spoilers will either add or reduce the car's downforce, meaning the combined effect of air pressure and gravity pressing down an object toward the ground and increasing its stability, and in the case of the cars in Grand Theft Auto V, reducing the object's airtime:

•Feltzer: spoilers add significant downforce.
•T20: active spoilers create significant downforce, damaged spoilers take away that downforce.
•RE-7B: stock spoilers have little downforce; prior to the Grand Theft Auto Online: Gunrunning DLC, custom spoilers gave significant downforce, but this has since been reduced.
•X80 Proto: spoilers add only a little more downforce on their own, but gets significantly more downforce when braking while the flaps are active.