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I didn't really see much change in the mileage. The overdrive just allowed me to drive faster. Same when I lost the overdrive. I went back to driving slower on the highway and with the slower speeds, mileage didn't drop much if any at all.

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If you put an overdrive in and then spin your motor up to the same rpm (going faster than your comparison speed) then no you wont see a mpg gain. But, if you go THE SAME SPEED then all things being equal the rpms will be lower and that is where you get the mpg increase.