Under certain conditions the taller tire in question can improve your mpg. My personal experience recently was 19mpg. I drove 350 miles each way to Denver and back. I bumped tire pressures on my D rated tires to 47 psi, I normally run them at 42psi. I ran the AC the whole way and was cruising at 70-75 mph. My taller tires lowered rpm and were likely key contributors to better milage.

My typical road trips are through the mountains, over several high passes, at lower speeds. I generally can make the 200 mile round trip to Durango and back in 4-4.5 hrs. I generally get about 16 mpg. I do not think my tires are helping or hurting me here, my right foot has more to do with it. This milage seems very typical compared to other owners with smaller tires. In town I am down around 12-13 mpg, again very typical for the truck.

My reccomendation for your truck is the Michelin LTX M&S. I would buy an LT tire in load range C. If it is availible, for your use I would run a 265/70-16 rather than the taller 75 series sidewall. This minimizes the hieght difference and provides a wider, more stable platform. These tires will easily go 60,000 plus miles and offer excellent traction and durability.