I just purchased a 2003 GMC Sierra 1500HD with 137,000 miles. I've been through two tanks of gas and I'm only getting around 10.5 mph city/highway. Is this normal or is something wrong?

 

Do the normal tune-up routine, spark plugs, wires, air filter. Are the tires stock height? My 07 2 door 2wd 2500HD only got 11MPG from brand new. I got it up to 16MPG consistently, even 18mpg quite often, did a quality cold air intake, exhaust, Blackbear performance tune and larger tires in the rear to offset my 4.10 gears. Now I am only getting 10mpg, but I also have a hot rod engine in it now.