Ah so that is kind of the point I wanted to try and make. The engine isn't any less efficient in lower gears. It's just that you are measuring MPG and not some

Ah so that is kind of the point I wanted to try and make. The engine isn't any less efficient in lower gears. It's just that you are measuring MPG and not some other metric like hours per gallon. If you are able to travel at 60 the entire time then sure, but at the same RPM your engine is consuming the same fuel in a lower gear. I'm pretty sure that MPG value would also very aggressively change if you were constantly accelerating and deceleration to hit that 60 MPH number whereas it would in fact have been more efficient to travel at a lower constant speed.