The max towing capacity for a newer 4.0 V6 is around 6,500lbs or 6,300. If you're towing anywhere near the max your gas mileage will be crap. I towed a 21' travel trailer loaded down with a weeks worth of hunting gear and supplies for two guys. My gas mileage was aorund 8-9 MPG. The truck handled it pretty well and I managed to hold over 55mph there and back. I averaged 65mph though but the hills slowed me down. I was flooring it 80% of the time though. That is with stock suspension and 31's. I normally get 20-21 highway and 17 in town to give you an idea. I would say if you tow a lot get a full size truck. If it's just occasional then get the Ranger. I'm happy with my Rangers gas mileage, others for some reason get terrible mileage and always complain.
05' 4x4, Auto, Locked
F&R, D44 SFA & 8.8 w/disc & c-clip eliminator , Yukon Chromo's F&R, Custom Skids, Sliders, Bumpers, 35" BFGs, 5.13 gears, Riddler & Solid Diff Covers, Superchips Flashpaq, CB, Optima Red Top, Gibson Exhaust, K&N Air Filter & a lot of other little things.