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Well, aerodynamic drag increases with the square of speed, so:
40 = baseline
55 = 1.375x higher. 1.375x1.375 = 1.89, or 89% more drag
70 = 1.75x. 1.75x1.75 = 3.06. THREE TIMES the air drag, or 62% more than 55.
Actual results vary. Higher speed and drag put more load on the engine, so it operates more efficiently than at low load, so the extra cost isn't as bad as this shows.
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11-mile commute: 100 mpg - - - Tank: 90.2 mpg / 1191 miles
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