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"....Guess I don't understand how fuel mileage will improve as the rpm increases ...."
Come on Dave every engine has a sweet spot. Too slow and the engine is lugging and much above that and the mileage also goes down. In the case of my Mustang and Plymouth with the cams in them the sweet spot is 2100 and 2600 respectively (a lot more cam in the HEMI). The tires and gears put that put those ranges at 75 MPH.
Back when the 55 MPH speedlimit first went into effect there was almost nothing out there with an overdrive and the gear ratios were relatively deep compared to today. Most of us were turning our engines over 3000 RPM at highway speeds. Nowdays 75 is often under 2000 RPM on many cars.
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Houston seems to be slowing down on its own.
I drive 65 (the speed limit) and WAS being passed all the time.
Now more and more people are driving at that rate.
