I remember something about aerodynamic drag increasing with speed, not being linear.
Something like drag increasing at the square of the speed??
It's real easy to see in the (brick-like) Escape. On a flat road at 35-40 MPH, it gets 40+ MPG..
But on the highway it starts dropping off..
It's going to drop down to around 26 MPG at 60MPH and likely be less than 20 mpg at 70 MPH..