I replaced my stock 235/70/R16 tires with 255/70/R16. How badly does this mess with my speedometer? I had the Ford dealership do it so I'm assuming they recalibrated my speedometer. Would such a small upgrade even make a difference? I noticed on what seems like every other F150 had the 255/70/R16. The 255's look better and fill up the tire well more.

 

You increased your tire size by about 3.4%, which will make your speedometer indicate low by that percentage. If your speedo says 40, you are actually going between 41 and 42. If the speedo says 70, you are actually going between 72 and 73.

This of course assumes that your speedometer was dead on to begin with, which might not be an accurate assumption. I usually suggest checking it against a GPS, (beg, borrow or steal one if you don't have one.) This will give you real-world data on how far off your speedometet truly is. It is possible that it was a little high before, and is now more accurate than it was. At least you will know, and then you can decide if correcting any error that might exist is worth the trouble and money.

I wouldn't bet a nickel that Ford did anything with your speedo.

 

Here's an online calculator
http://www.discounttire.com/dtcs/infoTireMath.dos