[jakobnev]

I'm thinking there are two kinds of wake at the extremes: One where a bubble is formed behind the car that has the same air in it all the time, the other where new air that has been accelerated flows into the wake constantly. (In reality you would always be somewhere between these extremes)

The first kind would of course be better since air would flow around the car and then around this bubble and be left behind not much disturbed and the second kind would leave behind a trail of fast moving-energy sucking air.

This effect could be measured, for example, in a wind tunnel by turning a smoke generator on and off and observing how long the area behind the car stays smoky.