[johnunit]

I think some clarification is required:



Are we trying to find a car that gets better mileage at 75-85MPH than at 55, or a car that gets better mileage at 75-85MPH compared to the average car?


A new corvette or other sportscar (maybe the Lambo mentioned earlier) would likely qualify for the former, while a Prius, Insight, TDI VW, etc. would qualify for the latter.


I think the aerodynamics+engine load+gearing are what would would make differences in relatively comparable cars.

For instance:

the BIG elbow in my boat of a summer car (85 Crown Victoria, dead flat front end, 5.8L engine, 4200lbs, etc.) is at around 70MPH. I lose maybe 10-15% efficiency driving 65MPH instead of 50, but I lose 20% or more driving 75-80 as opposed to 65. 70MPH is about as fast as I can justify driving unless I'm in an emergency and it's long-distance.

However, this is with a 2.73 rear gear. That means that in OD 65MPH is around 1500rpm. Idling practically. With the 3.55 axle ratio that is a popular performance/acceleration choice in these cars, the elbow is at more like 60MPH because of the higher RPMs and the increased pumping losses from needing less throttle in OD to maintain a given speed. Go to a 1992-97 model of the same vehicle, basically the same trans but an engine with a higher designed RPM range and a much more aerodynamic body, and the elbow is at 70mph-ish even with the 3.55 axle ratio.