Actually what happens when a car gets really close behind another (don't try this at home... no really... don't) is that the airflow over the two cars is no longer determined by the air surrounding the seperate cars, but by two cars acting as a single car... This will have two effects... One being that air is pushed forward by the trailing car, slightly pushing the car in front... The other being that the airflow over the rearwing of the lead car is not pulled down behind the car, but rather pushed backwards over the second car, leading to less drag and thus more speed. Most notably in NASCAR, you can see its effects when two cars are drafting close on a superspeedway and the lead car gets loose going into a turn... That's because the air is no longer hitting the rear spoiler, but moving over it towards the trailing car's windshield. Less downforce on the rear means watching your turning.