Ok heres a therory that Ive been pondering.. When I bought my truck a year ago my rear tires were at half tread, I drove from Indiana with the trailer, quad, jeep and another trailer (4x8) sandwiched inbetween the trailer and the jeep with the jeep loaded down and got 14mpg with CC and AC at 75.. After driving all summer and putting 20k+ miles on my truck Ive been only getting 15-16 unloaded on the high ways and my tires are just about bald.. Now betwwen new tires and my bald tires there is about 1-1.5 inches differance in height, (old stock to new stock sizes) That little difference will help to drop my RPMs alittle and gain more speed (compared to RPMs) which in turn may give me an extra 2mpg per gallon, and can bring me up to 17-18 or better MPG.. More surface (hieght, full tread) on tires = drop Rpms on the motor= better MPG..
I have been keeping track of my miliage and I will give a report since I will be getting new tires later today (stock size of course).. 6 of them to boot
Now Big tires (bigger than stock) can hurt on MPG because your making your motor work harder to get the tires turning, producing higher RPMs on take off and stop and go traffic.. On the highway it may help as long as its only 1 size larger than stock but you still sacrafice somthing for a gain.. Wider tires tend to cause a drag effect because your having to move alot more air and it creates Vortex's.. The added height of the vehical causes the same affect and you might as well kiss Fuel miliage away.... You can counter act these forces by changing the gears ratios and eliminate the strain of the motor for the over sized tires but you will still have the large drag effect in resistance, IE drag