To the third power actually.
Someone didn't take thermodynamics.
I've been tracking the data myself with torque. Fuel consumption per unit time increases with the square of speed because after 65mph drag does.
But since you get there faster it's just linear increase in consumption per unit distance.
Personally, I discovered my AFR drops from 14.1 to as low as 11.5 any time the engine produces over 400 grams CO2/mile for more than about 3 seconds. On a completely flat road this works out to about 85mph or so, I can do slight inclines at 80, and if I don't ever want to have to touch the cruise control, 75-76mph.
at 55mph I get 34mpg, at 70 I get 32, at 75 I get 29.5 and at 80 I get about 27.
If I want to get there I go 75.
If I kinda want to get there I go 65.
If I don't really care or I'm feeling cheap I go 55.