Slip steam only works well at extremely fast speeds. The distance required to slip stream is too small to be safe for highway driving. You might get some effect behind a transport truck, but not much.

How it works. As the 1st vehicle moves, it is pushing air away from the vehicle. This creates a small pocket of slightly less dense air behind the vehicle. If a 2nd vehicle can follow behind, in that pocket of less dense air, then it has less wind resistance slowing it down, giving it the slip stream advantage.