The tunes will really depend on what octane gas you are used to using... most people go for an 87 octane tow tune and an 87 octane performance tune. I have this and I recommend it because it covers most situations that I would find myself in... If i raced my truck I would get the 93 performance tune as well, but I would need to make sure that all the 87 gas was out of the system...
With the mileage, a tuner can only help. before you install it though I would make sure that nothing is wrong with the truck mechanically, clean the MAF and the TB as well, and maybe even change the spark plugs if it has been a while. Ford recommends to change them around 60-70000 miles, so theoretically you should have had them done once already. This could be a reason for the bad gas mileage as well.
change the oil as recommended, and the tranny and differential fluids as well, and keep up with any other recommended maintenance and you should be good. as a comparison, I have 35"ish tires as well and I just filled up and got 14 mpg's.
the best I have seen is 17.7 on the highway with a tailwind, and that was only once, I usually get 15.5ish on the highway.
hope that helps
