This calculator compares two tyre sizes in New Zealand, working out the overall diameter of each from its width, profile and rim, and showing the difference and how it would affect your speedometer. Tyre sizes are written as three numbers, such as 205/55R16, giving the width in millimetres, the sidewall height as a percentage of the width, and the rim diameter in inches, and from those you can work out the overall rolling diameter, which is what really matters when changing size. People plus-size their wheels, switch profiles, or fit a different tyre for looks or grip, and the key question is whether the new size keeps the overall diameter close to the original. If it does, the speedometer, odometer and gearing stay accurate; if the diameter changes much, the speedo reads high or low and the ride and handling can shift. You enter the width, profile and rim for your current tyre and a proposed one, and the calculator returns each overall diameter and the percentage difference, which is the same as the speedometer error the change would cause. Aim to keep the difference within a couple of percent for an accurate speedo and to stay within warrant of fitness rules. An estimate based on standard tyre geometry.
Keep the difference within about 2% for an accurate speedometer and to stay within warrant of fitness rules. An estimate from standard tyre geometry.
Each tyre's overall diameter is the rim diameter converted to millimetres (rim times 25.4) plus twice the sidewall height, where the sidewall height is the width times the profile percentage. The difference is the percentage change from tyre 1 to tyre 2, which is also the speedometer error the change would cause.
A 205/55R16 tyre is about 631.9 millimetres in diameter, and a 225/45R17 is about 634.3 millimetres. That is a 0.38 percent increase, small enough to leave the speedometer essentially accurate.
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