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As cars have become heavier, more aerodynamic and come with more powerful engines, the result is that the speed at which you get the best fuel mileage has risen.
It used to be around 30mph in many cars (looking at a graph from Glenn's Gas Guzzlers Guide from 1975). In my Hybrid Avalon it's closer to 55mph.
The thing is there is a transition from where the engine is more inefficient from being under too low of a load to where the aerodynamic drag is the major factor in fuel mileage.
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