As a car passes through the air it leaves a slipstream of lower pressure directly behind it, this is due to the fact that the air does not have enough time to make the pressure in front of the car equal to the pressure behind it.
The obvious method of reducing drag, would be to get a fan and blow
air pushed from the sides of the car to the back, so the air pressure behind the car increases and reduces drag, but this takes energy and so would be pointless although it would improve fuel efficiency slightly and aid acceleration and maximum speed.
Another method would be to replace the area of lower pressure air with a fin, though this fin would need to be able to change shape with the speed of the car and the car would end up as being extremely long.
You could of course have a v shaped car end in order to reduce drag, the v shape would reduce the extremes of low pressure behind the car experienced, this would mean a shorter car, but you would still experience drag.
If a fan were to be combined with this short v shaped car end in order to negate the air resistance only experienced during high speed, it could be calibrated to blow harder when air speed increases and need less energy due to the v shaped car end.