> after spending a week driving at 70 the difference in mileage was almost unnoticeable.

In dollars, the additional amount you pay to drive an ICE faster is considerable:

https://afdc.energy.gov/conserve/behavior_techniques.html#:~...).

"For light-duty vehicles, for example, every 5 mph you drive over 50 mph is like paying $0.18 more per gallon of gas (based on the price of gas at $2.63 per gallon)."

Even that is a simplification. Air resistance increases exponentially with speed, so the jump in resistance (reduction in efficiency) is much greater from 70-80 than from 50-60.

Also, gas is a lot more expensive now, so the additional $0.18/gal could easily be doubled or tripled at today's prices depending on where you live.

> It helps that I have a small, aerodynamic car that's geared high instead of a truck or blocky SUV.

According to consumer reports, even a very gas miserly Toyota Yaris loses 4mpg going from 65-75mph. It's probably double the efficiency loss if you go to 80mph. Can't cheat physics.

https://www.consumerreports.org/cro/news/2009/09/tested-spee...

I find that very hard to believe, given that drag increases with the third power of velocity. By almost unnoticeable do you mean that you didn't notice it, or did you robustly measure and compare?

It's entirely possible, even likely, the lack of difference is due to age-related efficiency losses at lower speeds rather than not losing efficiency between 70 and 80. There's also the fact I'm running AC on max in very hot weather, which maximizes the loss due to AC and increases the amount I save by shortening the trip and thus reducing the amount of time the AC is running.

My overall trip efficiency maxing out at 70 was 31mpg, and 80 was 27.9mpg.