As mentioned in an earlier post: Time is valuable. Over a year of driving slow, it's likely 80 additional hours are spent in the car for a typical driver. That's 2 work weeks, lost to the highways. Placing a value on the fuel saved vs. time consumed, it calculates out to be in the neighborhood of $4/hour.
Put another way, my salary is an order of magnitude more than that. Also, my free time is exceedingly valuable to me. It's why I work, to have the funds to spend my time doing what I want.
The performance of an ultra efficient car can be used to save fuel at high speeds. My Jetta TDI returned a solid 42MPG at 80, day in and day out.
Klaus Savier sets all sorts of aviation efficiency records. One that stands out is: 2 people, 45 mpg @ 207 MPH and 60 mpg at 160 MPH. And California to Florida on 26 gallons, in 9 hours.
https://www.wired.com/2009/11/45mpg-at-207mph/
My Cessna 177RG can achieve 25 mpg at 125 mph, if I try. High speed MPG is all about aerodynamics.