The factory bolts a small steel tag onto the axle's differential cover at its 9 o'clock position. Look on that tag, you'll find your axle ratio.
This axle tag is indicating the axle has a 3.73 gear ratio inside.
Something that can significantly reduce the mpg is a bad upstream O2 sensor. The upstream O2 sensor provides information to the computer so it can determine how to set the air-fuel mixture. When the upstream O2 sensor goes bad, and they do go bad/wear out just like spark plugs do, it can give bad data to the computer, fooling the computer into thinking the air-fuel mixture is too lean. The computer then keeps adding more & more fuel trying to get the air-fuel mixture correct and eventually the air-fuel mixture is full rich which will destroy the mpg as well as performance. Sometimes you can even detect the strong odor of gasoline from the air-fuel mixture being too rich.
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