Our 05 5.3 tahoe 4x4 would get @15 mpg. It had 35s, 4.56 gears, an air intake, exhaust used premium fuel and a 'programmer'. Driving was mainly 45 mph city areas with light traffic. Highway time would reduce mpg.
Years before that we had a 97 f150 5.4, that would get 14. 4x4, 38s, 4.56...