I put a fan blade in my air intake, a magnet on my fuel line and pre-heat my fuel. The truck is actually producing gasoline now. I have to stop from time to time and pump the extra gas into somebody else's car to keep the generated fuel from spilling on the road.
OEMs would use these devices, but fuel economy means very little to them. The several cents they would pay for fan blades and magnets in bulk would not be worth it because consumers have no interest in better gas mileage.