Original Tire
New Tire
Calculations assume a stock instrument cluster calibrated to the original tire size. Tire inflation, load, and wear affect actual rolling diameter.
Every time you change tire sizes, your speedometer and odometer get recalibrated — just not intentionally. The instrument cluster in your vehicle was programmed for the original tire's rolling circumference. Swap in a different size and the math is off by whatever percentage the diameters differ.
The effect scales linearly: a 3% larger tire means a 3% speedometer reading error at every speed. At 60 mph indicated you'd be doing 61.8 mph actual. At highway speeds that gap is real. It also means your odometer accumulates fewer miles than you've driven, which understates wear on service-interval components and can affect warranty claims.
The calculator above gives you exact numbers for any two tire sizes. Enter your original size and the size you're considering to see the full speed table and odometer impact before you buy.