My ext-cab pickup with 3.0/auto gets about 13 city and 16 highway and has since 1993. It has remained constant through 3 gearing changes, a 4-5" suspension lift, a 1" body lift, and tire changes (31"-33"). The best it has ever gotten was 17-18 range but that was at high elevation in Colorado (thin air has less wind resistance).

93 4X4 ext-cab, auto, SR5, 3.4 V6, supercharged, 2.1" pulley, URD fuel mods, Aquamist WI, IPT valve body mod, dual cases, 4" superlift, Alcan springs, 33 BFG MT, ARB locked front & rear, 5.29 US Gears, RB 1" BL, 1.5" BJ spacers, TJM T-17, Warn m8000.