Drag (wind resistance) is the biggest factor in fuel economy at anything over about 50-ish mph...and I believe it's an exponential quantity. I can't remember the formulas anymore, but all the other parasitic losses are quite small in comparison at elevated speeds. I'm sure there's a fuel economy guru who can speak to it in more detail than myself, but you get the idea.
With my EB, if I go from 62mph to 72mph on the highway, my economy goes from 20.5+, down to 18-ish with mine.
And given that trucks punch such a big hole in the air, it's little surprise they're so greatly affected.