For a normal vehicle that might be true if the difference in the speeds was say 60 vs. 55 but for 55 and 75 the relative velocities for a given Cd preclude that.
"For a normal vehicle that might be true if the difference in the speeds was say 60 vs. 55 but for 55 and 75 the relative velocities for a given Cd preclude that"
If a given vehicle gets better mileage at 75 than it does at 55 I'd say there's something seriously wrong with either its design or state of tune. Crappy old auto tranny designs that wouldn't lock up their torque converters until a certain speed was reached in top gear, that sort of thing. Or failing to upshift when they should. Or with such poorly chosen and widely spaced ratios that 2 is too low and 3rd too high at 55. It could happen but it's unlikely in any modern car in which all the manual trannies are at least 5 speeds and all the auto trannies are at least 4 speeds (my '04 Accord has a 5 speed auto, and I think I've read that the new Jetta is available with a 6 speed auto).