Apologies for spreading potential misinformation, I too am having trouble finding a definitive source for what I thought was common knowledge.
I did some light research and was left with these notes:
Air resistance increases at higher speeds by the inverse square law, more aerodynamic cars help offset the wind load but physics always wins.
Peak engine efficiency is at the highest gear and approximately 1500 RPM, a more powerful engine may hit this at higher speeds.
The EPA now tests various speeds up to 80 MPH when calculating highway fuel economy, so it wouldn’t be surprising to see manufacturers optimizing across the range.
Cars that are optimized for the above points may get the best gas mileage at 60 MPH, but will have less MPG loss at slightly higher speeds.
90 MPH will never be more efficient.
Personal anecdotal evidence has too many variables and I don’t have access to a variety of sports cars and straight level sections of highway with controllable wind settings to confirm.
Can you provide an example? In all data I can find, even for sports cars, they still have the best mpg at speeds significantly lower than 75mph.
Apologies for spreading potential misinformation, I too am having trouble finding a definitive source for what I thought was common knowledge.
I did some light research and was left with these notes: