seems to me there are 3 major uses of fuel.
One is overcoming aerodynamic drag.
One is overcoming rolling resistance.
One is accelerating the vehicle.
Of the three, I suspect the middle one is the least significant.
I agree aero drag would be MUCH easier to calculate at higher speeds.
Also, I suspect some of your curve is actually from the wheels energy being non linear with respect to speed  the wheels are rotating, so they give energy to the system as a function of the square of the mph.
accelerating the vehicle is a function of rotational inertia and weight of the vehicle.
