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.