At 50 mph, the Cruze Eco requires about 9-10 horsepower. Extrapolating from this:At 55 mph, it would need roughly 11-12 hp
At 65 mph, it would need about 18-20 hp
At 75 mph, it would need approximately 28-30 hp
And since the hp requirements are small the difference in fuel economy is small
For example, I have driven the same stretch repeatedly at around 55 and also around 70.
Only 3-4 mpg difference in my van.
—”Yes, but the actal horsepower requirements to overcome air resistance are surprisingly small”
Soeed squared is a simplification of air resistance; where most of your HP is used at highway speed.
Air Resistance Proportional to the Square of the Speed!
Even with your wizzer running richer than D.J. Trump at 55, with raw gas pouring from the exhaust and lean as Jack Sprat at 70 and pinging a hole in your piston; you will not be burning less fuel.
There are more than a few online tools to approximate for horsepower/speed.
“For all you Bonneville Salt Flat people or wannabes
This equation will show you how much Horsepower it
would take to run those high speeds.”
http://www.wallaceracing.com/Calculate%20HP%20For%20Speed.php
The drag coefficient for many cars is readily available.
https://en.wikipedia.org/wiki/Automobile_drag_coefficient
And the Epa specs for speedometer accuracy is a JOKE at 10% !!!
You probably can not break the speed of light (it is the LAW) and you will never see better MPG at 20MPH faster speed.
Dream on...