Here is what I have seen when I followed a Beetle in the rain, it stood out like crazy, these 2 very active vortices. The thing to keep in mind about these devils is this, the air was not moving (Let's assume a calm day here) before the car rams through it, but at the back of the car, you have created 2 horizontal tornadoes......these do NOT follow you.....This is the tough part to understand, you are constantly generating them, it requires power all the time to keep the air moving like that. Imagine how much energy it would take to spin two 30 inch tornados constantly using say, a fan. It would be a fairly large fan and some large motors driving it. This is the energy your engine must create to keep those going, thus, less MPG.
I hope that makes sense.
