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I've tried that with a lot of different vehicles. Once you exceed the optimum diameter, your mpg starts to go down. Also, I've found on the Smart Car that it gets the very best mpg at 25mph in 4th gear - not 5th. That tells me that the car is very close to perfectly optimized already, so any changes should be small or, at least, obvious - like fender skirts or chopping.
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