I went from the factory 275/55/20s to 33x12.5x18s and the tire height in person was a lot closer than I originally expected. According to my GPS, my speedometer is only off by about 1 MPH when traveling at 70 MPH, so I did not bother to recalibrate. I have the Xcal2 with my VMP tunes, so I may change it the next time I reprogram.

My mileage dropped from an average of 15-16 MPG to 13-14 MPG, so I estimated a 1.5 MPG drop since adding the 20 lb heavier wheels and tires.

KR-Texas

 

i understand the theory behind the MPG decrease, but Ford's MPG spec on the truck doesn't change regardless of what wheel tire combo your truck has, a la 18" vs. 20".

I just picked up a set of OEM 20's with the stock pirelli's and i'm switching from the 18" FX4's with stock BFG rugged trails. Overall tire diameter is about the same, and I'll try to weigh them before I switch. Should I expect to lose some mileage?

 

u r probably right in what u expect

 

Planning to sell the FX4s? PM me if so.