I had a 2004 GMC Sierra SLT E. Cab Z71 5.3L with all options. In the 40,000 miles (4 yrs) I had it, I averaged 17.3 mpg. I logged all my miles and fuel for business reasons so I could easily compute the mileage to separate personal use.
But, the truck was 100% stock with oem tires, oem exhaust etc. Only thing I did was install synthetic lubricants all over. The truck was about 50% hwy / 50% short around town driving but no traffic lights (in this semi-rural area) to drag race from.