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I know it doesn't make sense, so please don't shhoot the messenger, but EPA requirements and poor fuel quality are to blame, ethanol that is in fuel now is les effecient and it takes twice as much ethanol to produce the same energy as the same amount of gas.
That is likely why you have seen a decrease in your mileage on your metro. Other epa requirements and standards for clean emissions also contribute to this along with cars being heavier due to safety requirements.
A new Honda can get 36mpg witha heavier car and more power than your metro has.