Hi there!
Some days ago I made about 30 miles at 90km/h, 55mph, behind a truck and it was just impressive, I've never tried it.
My maximum average on that road was 3.5l/100km, but behind the truck I made 3.2l/100km, with no aero mods at all. Just have over inflated tyres.
As you can see here:
I'd imagine if the car had an inferior Cd....