Manual transmission and proper gearing.
take that top off. but before anyone says anything im cover my tracks yes taking the top off reduces weight = better mpgs but then you have the air pushing against the tailgate either drop the gate or put a tonneu(spelling) cover on. 18 is decent for the 3.0 i got 20 once in mine 3.0 5speed 3.73 gears 235/75R15 wrangler radials and that was with easing it everywhere i went and 65-70 for 200 miles without passing any one but the occasional semi.