Well I kept the truck. I put on a FlowMax muffler and a K&N air filter and that seems to have boosted the mileage up a bit over 10MPG. I also measured the 31X10.5 tires and found that they are about 10% larger than the stock 235/75x15 that Ford reccommends, so I can factor my mileage up another 10%.
I plan to replace the large tires (they are almost new Michelins!) with 255/70x15 to get the rear end ratio back closer to normal, which should make it pull easier my trailer and maybe require even less gas.
I do love this Ford truck... I've never had one so nice. I am thankful.
The Area's Low Price Leader