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14.2 mpg highway & no error codes or lights on 2004 Buick Rainier

fuel mieage on local & highway, using high or low grade fuel, increasingly getting worse. Bought from dealer 1 year ago doing 16.0mpg, now down to 14.2mpg. no signs anywhere. mechanics won't look at it without a code to trace.

Asked by for the 2004 Buick Rainier
Check this government website for 2WD Rainier. http://www.fueleconomy.gov/feg/findacar.htm

They are not brilliant for MPG but some tires even can help fuel economy, (less rolling resistance). I wonder on a scan tool what the fuel trim figures are Long Term Fuel Trim figures show if your car's computer is adding or taking away fuel which may not yet have set a code.
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you may want to seek professional help because my 2006 AWD Rainer needed the stabiltrac serviced and after that the fuel mileage went from 15.7 to 19 mpg highway