Low Gas Mileage on Mud Tire

Hi! New to the forum here and I have a question. I bought a used 2008 Tacoma 4x4 double cab, V6, that came with 285/70/17 ProComp M/T Extreme tires on Eagle alloy wheels. Also has a front leveling kit that raises the front up a little. This truck gets a lot of looks and I like the looks too; however, it gets 10 mpg all around driving. I thought the V6 was rated about 15/19. I've heard the low mpg is due to: 1) the mud tires 2) the size of the tire, 285 vs 265/65/17 which is the factory recommended tire. This makes sense to me since a 285 tire is wider and more resistance? I've thought about getting a 265/70/17 or 265/65/17 tire to increase the gas mileage. I'd appreciate any thought or suggestions on switching tires. Thanks for any input!

 

Also the bigger tire throws the MPG off on your calculations...