Tire Size Comparison & Speedometer Error Calculator
Compare two tire sizes in standard width/aspect/rim notation to see the change in overall diameter and the resulting speedometer error when you switch.
Last updated: May 2026
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About this calculator
A tire's overall diameter is the rim diameter plus twice the sidewall height. The rim is given in inches, so it is converted to millimeters by multiplying by 25.4, and the sidewall height is width x aspectRatio / 100 (aspect ratio is the sidewall as a percentage of width). Doing this for both tires gives oldDiameter and newDiameter in millimeters. The speedometer error is the percentage change in diameter: (newDiameter - oldDiameter) / oldDiameter x 100. Because your speedometer is calibrated for the original diameter, a larger new tire travels farther per revolution, so the speedometer reads slower than your true speed (a positive error), while a smaller tire makes it read faster than reality.
How to use
Compare a 225/45R17 tire with a 235/40R18. The original diameter is 17 x 25.4 + 2 x 225 x 45 / 100 = 431.8 + 202.5 = 634.3 mm. The new diameter is 18 x 25.4 + 2 x 235 x 40 / 100 = 457.2 + 188 = 645.2 mm. The change is (645.2 - 634.3) / 634.3 x 100 = about 1.7%. So at an indicated 60 mph you would actually be traveling about 61 mph. Stay within roughly 3% to keep the speedometer and any electronic systems accurate.
Frequently asked questions
How much does changing tire size affect my speedometer reading?
Your speedometer assumes the original tire diameter, so the error equals the percentage change in overall diameter. A tire 2% larger makes the speedometer under-read by about 2% (you are going faster than it shows), and a smaller tire makes it over-read. Most experts recommend keeping any tire size change within about 3% of the original diameter so the speedometer, odometer, and safety systems stay reasonably accurate.
How do I calculate the overall diameter of a tire from its size?
Take the size in width/aspect/rim form, for example 225/45R17. The sidewall height is the width times the aspect ratio percentage: 225 x 0.45 = 101.25 mm. The overall diameter is the rim (converted to mm by multiplying inches by 25.4) plus two sidewalls: 17 x 25.4 + 2 x 101.25 = 634.3 mm. This calculator does the conversion for both tires so you can compare them directly.
Will a plus-size wheel and tire change my gearing or fuel economy?
Yes, indirectly. A larger overall diameter raises your effective gearing, so the engine turns slightly slower at a given speed, which can marginally help highway fuel economy but hurt acceleration; a smaller diameter does the opposite. The effect tracks the same diameter percentage this calculator reports, so a 2% larger tire behaves like roughly 2% taller gearing. Large changes can also throw off the odometer and transmission shift points.