Why would someone inflate their tires beyond what is suggested for simply a small mpg gain? I can think on a number of negative effects that would cancel out any mpg gain.
-- Uneven tire wear
-- Higher tire temps leading to more wear and increased possibility of blow outs
-- Increased possibility of accident due to less traction because the tire doesn't have an even patch of tread on the ground (more on the middle, less on the sides)
-- Increased wear on suspension and higher possibility of bending a wheel due to less energy being adsorbed by the tires (harsher ride)
There might be others but these are all I can think of for the moment
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