[aerohead]

I'm also in the "it depends" camp.I think every vehicle has a sweet spot.There's no question that loads increase with speed,however,how the powerplant reacts to increased load may be counterintuitive,as in my case with the Dodge D-100.It's mileage is better above 55mph,so in it's particular case,it's brake specific fuel consumption hits the "sweet spot" at 65mph,not true for any of my other vehicles.Experimentation may be the only means to discover peak efficiency for each vehicle.