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TireMath
Decode a tire size (e.g. 225/45R17) into overall diameter, and see how a size change affects your speedometer.
225/45R17 means: 225mm wide, sidewall height is 45% of that width, mounted on a 17-inch rim. Change any of these and the overall rolling diameter changes, throwing off your speedometer slightly.
Original tire (width / aspect / rim)
New tire (width / aspect / rim)
What you entered
Original tire diameter
17 + 2 × (225 × 45% ÷ 25.4)= 24.97 inNew tire diameter
18 + 2 × (235 × 40% ÷ 25.4)= 25.40 inDiameter change
(25.40 − 24.97) ÷ 24.97= 1.72%Actual speed at new tire size
65 × (25.40 ÷ 24.97)= 66.1 mphResult
Actual speed: 66.1 mph
Swapping to a 235/40R18 tire changes the rolling diameter by 1.72% — at an indicated 65 mph, you're actually going 66.1 mph.