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Tire Size Comparison Calculator

Compare two tire sizes and your speedometer error

How it is calculated

Enter the three numbers from each sidewall marking, such as 225/45R17: section width in millimetres (w), aspect ratio in percent (a) and rim diameter in inches (r). The overall diameter in millimetres is D = 2 × w × a ÷ 100 + r × 25.4: two sidewalls plus the rim. The tool then reports the percentage difference (D2 − D1) ÷ D1 × 100.

It also shows a speedometer figure, 100 × D2 ÷ D1: the actual speed, in km/h, when the original-tyre speedometer reads 100 km/h. A value above 100 means the new tyre is taller and you are going faster than the dial says. All six values must be positive. The tool only uses geometry; it ignores load index, speed rating and tyre deflection.

Worked example

Compare 225/45R17 with 235/40R18. Tyre 1: 2 × 225 × 0.45 = 202.5 mm of sidewall, plus 17 × 25.4 = 431.8 mm of rim, so 634.30 mm. Tyre 2: 2 × 235 × 0.40 = 188 mm plus 18 × 25.4 = 457.2 mm gives 645.20 mm. The difference is 10.9 ÷ 634.3 = 1.72%, so when the dial reads 100 km/h you are really doing 101.72 km/h.

Try this example →

Common mistakes

  • Putting the rim diameter in millimetres. It is in inches (17, not 431).
  • Entering the aspect ratio as a decimal (0.45). It takes the percentage as printed on the tyre (45).
  • Swapping the old and new sizes; then a taller new tyre looks like a negative error and the speedo figure falls below 100.
  • Thinking a positive difference means the speedometer reads high. It reads low: the car is travelling faster than indicated.
  • Judging only by width. Two sizes can differ widely in width yet have almost the same diameter if the aspect ratio compensates.

About this calculator

This tool compares two tire sizes written in the standard section-width/aspect-ratio R rim-diameter format (e.g. 225/45R17) by converting each to an overall diameter in millimeters: sidewall height = width × (aspect ÷ 100), doubled and added to the rim diameter converted to mm (you enter the rim in inches; 1 inch = 25.4 mm). The percentage difference between the two diameters is then reported as your speedometer error, since a speedometer is calibrated for the original tire's revolutions per mile.

Drivers switching to a different tire or wheel size — going up to a larger rim with a lower-profile tire, or fitting a "plus-size" set — use this to check that the new diameter stays within the ±3% most manufacturers consider safe for ABS, traction control and odometer accuracy. It's also standard practice among off-roaders fitting larger mud or all-terrain tires, who need to know how far the speedometer and odometer will now read from their actual speed and distance: below it with a larger new tire, above it with a smaller one.

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Comments (1)

  • Editorial team

    Half the diameter difference is how much the car's body height changes, since the wheel centre moves by the radius. For 225/45R17 against 235/40R18 the diameters differ by 10.9 mm, so the body sits about 5.5 mm higher. The speedo figure is the true speed when the dash shows 100, here about 101.7. Diameter is not the whole fit question; clearance matters too. Which sizes are you comparing?

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