Tires and speedometer

I am getting ready to buy a 05 F-150. The tires are not stock. The stock is like 255/something/17.The ones on this truck are 275/65/17. How much will that throw the speedometer off?

255-70R17 and 275-65R17 are 31.3" in diameter. 255-65R17 are 30.3" in diameter. Tirerack lists the 255-65R17 as a possible size for a 05 F150. This 275 tire is about 3.5% bigger in diameter as the 255-65R17 tire.

Check tire size on the door placard or go to and enter the make, model, and style of the F150 (i.e. XL, XLT, reg cab, super cab, etc) to find the stock tire size.

Ed B. is a free online calculator.

Don’t assume that your speedometer is accurate. Most are intentionally fast to help keep you legal. When your speedometer says 55, you are likely only going 50 mph. Most GPS’s display a very accurate speed and miles. You can also use a watch and the small white mile markers on the side of the interstates.

Correction. They are like 265/70/17. Looked at them wrong I guess. Getting the back tires replaced later today.