Tire Size Calculator
Compare the overall diameter of two tire sizes and estimate the resulting speedometer error.
Enter your values to calculate your result.
Formula
Sidewall Height = Width × (Aspect Ratio ÷ 100) Overall Diameter = Rim Diameter + 2 × (Sidewall Height ÷ 25.4)
A tire's size markings (e.g. 225/50R17) give its tread width in millimeters, its aspect ratio (sidewall height as a percentage of width), and its rim diameter in inches. Converting the sidewall height to inches and adding it twice (top and bottom of the rim) gives the tire's overall rolling diameter — comparing two tire sizes' diameters shows how much a tire change would throw off your speedometer and odometer readings.
Example
A stock 225/50R17 tire (about 25.86 in diameter) compared to a 235/45R18 tire (about 26.33 in diameter) is roughly 1.8% larger — meaning your speedometer would read about 1.8% slower than your true speed.
Frequently Asked Questions
Why does changing tire size affect my speedometer?
Your speedometer is calibrated for a specific tire diameter. A larger tire travels farther per wheel rotation, so the vehicle actually goes faster than the speedometer indicates for the same rotation speed — and a smaller tire has the opposite effect.