Your fuel economy will drop when you install
26 inch rims 5 lug and tires on your truck. They require more energy to roll on the street. Especially if they are off-road tires with big treads, you can expect to lose a few mpg with these tires. The bigger the tire and truck wheel size, the worse the fuel economy will get. So, if you do this upgrade, you will not lose a lot of fuel economy. But if you go overboard, you will lose more in this aspect. It will wear far more than skinnier factory tires.