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Was there really any change before they repealed the 55 MPH national speed limit? No, not really. Vehicle aerodynamics and engine efficiency both started to improve considerably sometime in the later 1980's. There was pressure from all around for cars to get better mileage. Gas prices went up so people wanted cheaper transportation, and the government also started caring about emissions. The whole transition was fairly gradual.
The optimal speed for good mileage varies from car to car. Going slower isn't always better, and some vehicles get their best mileage at surprisingly high speeds. (My buddy's VW Corrado seems to get it's best mileage at between 75 and 85 MPH.) Many very sporty cars have good aerodynamics and always have, but surprisingly many of the less sporty looking cars have almost equally good aerodynamics these days. I think what really happened is that the government realized that the limit was ultimately unenforceable, had a negative impact on the mileage of many cars, and wasted people's time. I think the only substantial change was that manufacturer stopped highlighting the 55 MPH mark on speedometers.
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