14.2 mpg highway & no error codes or lights

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.

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