Randy_the_Hack -
Welcome to EM! I looked at the EPA website but I couldn't find your car. I did find a CNG version that get's a little over 10 MPG combined :
2004 Chevrolet Silverado 2500 HD 2WD (CNG)
http://www.fueleconomy.gov/feg/calcu...D%202WD%20(CNG)
CarloSW2