Stupid question, but: especially if they don't call for the "standard" 30-35psi, are they inflated properly? Running 32psi instead of, say, 40 could easily drop mileage 5-10%.
I had 2 front tires put on my old J-20 pickup, and noticed it was handling (term used loosely when referring to an old Jeep pickup, but anyway...) kind of mushy. I knew the tires were good (the venerable BFG All-Terrain), so I stuck a gauge on the valves. Instead of the correct 50psi, the tire shop had only put about 32psi in them! When I returned to the shop, they saw nothing wrong with this & refused to inflate them properly. They also charged me $5 for new valve stems, despite my J-20 having metal screw-in stems that do not need to be replaced.
I no longer buy tires (or anything else) from these yahoos, needless to say.
1984 Ford B-700/ThomasBuilt bus conversion