Tyre size comparison calculator
The general case: any two sizes, with the percentage difference that decides whether the swap is safe.
mm
%
inches
mm
%
inches
Diameter difference
2.980%
- Original diameter
- 634.3
- New diameter
- 653.2
- Ride height change
- 9.5
- True speed when the dial reads your speed
- 102.98
- Odometer error
- 2.980 %
- Original circumference
- 1992.7
- New circumference
- 2052.1
- Within tolerance
- 1
Show the working
- Original width = 225
- Original aspect ratio = 45
- Original rim = 17
- New width = 245
- New aspect ratio = 40
- New rim = 18
- Indicated speed = 100
- Original diameter = 17 * 25.4 + 2 * 225 * 45 / 100 = 634.3
- New diameter = 18 * 25.4 + 2 * 245 * 40 / 100 = 653.2
- diff = 653.2 - 634.3 = 18.9
- Diameter difference = 634.3 > 0 ? 18.9/634.3*100 : 0 = 2.979 66
- Ride height change = 18.9 / 2 = 9.45
- True speed when the dial reads your speed = 634.3 > 0 ? 100 * 653.2/634.3 : 0 = 102.98
- Odometer error = 2.979 66 = 2.979 66
- Original circumference = pi * 634.3 = 1992.71
- New circumference = pi * 653.2 = 2052.09
- Within tolerance = abs(2.979 66) <= 3 ? 1 : 0 = 1
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- diameter = rim x 25.4 + 2 x width x aspect / 100