Not all 'wings' are actually spoilers. They make them for vans and pickups that aren't used to spoil the lift generated by the shape of the vehicle, but to actually direct airflow into the wake behind the vehicle. That decreases the gas used by reducing the 'pressure drag' (God, I can't even remember the real name... thats disturbing).

By having a airfoil on the back of the cab of a pickup, the air is also directed up and over the bed. That also reduces drag (and increases gas mileage), much like either a cover over the bed or those ugly-ass net tailgates.

Its cool when people talk about stuff I actually know

Edit: I actually took the time to dig out one of my aero books and look it up - form drag. Lame, huh (me, not the name )?