May be a little, but classic hypermiling technique is to do that anyway. So the slipstream gains are real since absent the truck a hypermiler is going to do the same thing anyway. I never hit the heady heights of good hypermilers, but I had a car I could reliably do 23 km/l, 55 mpg in.

Funny, my current car does 24 mpg on the highway at the speeds I like. Half the efficiency and probably mostly due to me.