I bought a 2002 Chevy 2500HD ext cab. It has about 1200 miles on it now. Most of my driving has been in town and without towing. I'm lucky if I get 11mpg. My buddy just bought the same rig in a crew cab version. He has approx 900 miles and has been driving mainly highways. He claims he is getting 17-18 mpg. But I think he just has trouble doing simple math . Any imput ?