kayt425

Hi everyone, I am fairly new and I am not a knowledgeable truck guy. I recently bought a new 2011 FX4 Ecoboost SCrew and i put some aftermarket wheels and tires on it. When i put the aftermarket wheels and tires on it my MPG dropped drastically. I am now getting 17 MPG on the highway at best and 13 around town. So my question is what causes the dramatic drop in MPG? I apologize for my ignorance but i would love a good explanation...

I am now running 33" Toyo AT tires and 20" XD monsters. Tire size is 275/65/20

I am shocked that i have seen such a huge drop. Will my Truck Computer not calculate mileage correctly now with the change in setup?

Thanks in advance...

Steve

hillwood24

The weight of the new wheels/tires. You'd be surprised how big of a difference this can make.

10-Ford

To get an accurate MPG calculation you have to update the tire size in the computer.
This is usually done via a tuner, I think the dealership can do it as well but not sure.

As an example, if you had stock 20" tires, 275/55/20 and went to 275/65/20 your tires are turning about 40 revolutions less per mile so the computer doesn't calculate you going as far as you really are on a gallon of gas.

This also screws with your speedo, at an indicated 65 MPH you're probably traveling at about 69 MPH.

Discount Tire Direct has a tire size calculator in their info center that can give you approximate differences between your old and new tires.

TopGun

just divide your new tire circumference by your old tire circumference, then multiply that number by what your computer mpg reading is to find your new mpg. to find circumference use the equation (diamteropie=circumfernce) pie=3.14.