I need a disagreement settled. Two 2007 F150s both with their factory shoes on. One has 17s and the other 18s. I say that there's no special calibration anywhere regardless of the size wheels that came on it out of the factory. Am I right? My dad is a little old-fashioned and would love the look of my 18s on his XLT but is convinced there's some computer calibration that would have to change.

 

Tire size is the only thing that will effect it. As long as the tires are pretty close to the same diameter, I wouldn't worry about having the speedo recalibrated.

 

There is a difference, but it isn't anything very different. The difference between 255/70/17 and 275/65/18 is usually a hair under 100mm

 

Get the height of both tires, or get the circumference of both tires. Which ever you use, deduct to get the difference and divide what you you get by the smaller of the two heights or circumerences. That will give you the percentage your speedo will be off. If they happen to be the same, then it will not be off.

 

Inside the passenger door tells you what the vehicle was calibrated for. I had posted a clculator fro this before. You can goto the "tirerack.com and see what their calibrator says. I went from 235/75/r17 to 265/70/R18s and I am now 3.8 mph over what my speedo says. i used my gps.

 

I found the wheel calculator site for ya:

http://www.dakota-truck.net/

then scroll towards the bottom and look for wheel calculator