sluggish engine

I have a Ranger xlt std cab, 3.0 v6, 4x4 truck that I recently purchased. When it had stock tires it got about 14 mpg which is pretty bad. Then I bought some 31x10.50 mudstars and put them on the truck. I was expecting worse gas mileage but now its really bad. probably like 10-11 mpg. engine has...