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Q: bad gas milage on 2002 Chevrolet Silverado 1500

it went from getting 18 to 19 mpg. down to around 12 mpg. has anybody had this happen and found what caused it?
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Assuming you are checking your mileage at highway driveing it could be several differant things that could cause a decline. A lazy O2 sensor or a coolant thermostat that is staying open causing engine temp to run lower are things I look at first. Air filter or dirty mass air sensor can cause low fuel mileage. Of couse the basics should be checked first but cannot rule out a brake caliper draging or transmission that may not going into overdrive. Im sure someone else will comment on things I have failed to mention.
My 2002 Silverado went from 20 MPG to 16 MPG as soon as they started adding 10% ethenol to the gas.
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