In your link, the maximum fuel efficiency curve shows a nearly flat region between 35 and 55 miles per hour. There are many contributions to the curve.
1) Air drag force is proportional to velocity squared. the power scales as velocity cubed.
Air drag depends on the frontal area of the vehicle, the drag coefficient, and the air temperature (and elevation).
2) The engine is most efficient at about 80% of max torque, and 35% of redline.
3) all other major losses (e.g., tire rolling friction coefficient) are linearly proportional to velocity.
4) For automatic transmissions, subtract 15% from your mpg.
5) If you have an automatic w/ torque converter lockup, use it.
So, derive the equation for air drag.