Tire diameter, sidewall height and revolutions per km from a size such as 225/45R17, and the speedometer error when you change tire sizes.
The new 235/40R18 is 1.72% larger than the current 225/45R17 (645.2 vs 634.3 mm). When the speedometer shows 100 km/h, you are actually doing 101.7 km/h.
The speedometer counts wheel turns, so a taller tire covers more ground per turn than it was calibrated for.
A metric tire size such as 225/45R17 gives the tread width in mm (225), the sidewall height as a percentage of that width (45%), the construction (R for radial) and the rim diameter in inches (17). The overall diameter is rim × 25.4 + 2 × width × aspect ÷ 100, which for 225/45R17 is 431.8 + 202.5 = 634.3 mm. Circumference is π × diameter, and revolutions per km are 1,000,000 mm divided by the circumference.
People compare sizes before fitting larger wheels, winter tires or a different brand. Moving from 225/45R17 to 235/40R18 makes the tire 1.72% taller, so when the speedometer shows 100 the car is doing about 101.7: the speedometer counts wheel turns, and each turn of the taller tire covers more ground.
The figures are nominal sizes; tread wear, low pressure and load make the rolling diameter slightly smaller. The load index and speed rating after the size, such as 94H, are accepted and ignored.
Checked against: Python decimal: 17×25.4 + 2×225×0.45 = 634.3 mm; 18×25.4 + 2×235×0.40 = 645.2 mm; 10⁶/(π×634.3)
Checked against: Identical diameters give no error
Checked against: Python decimal: 16×25.4 + 2×129 = 664.4; 15×25.4 + 2×126.75 = 634.5; 10⁶/(π×634.5)
Checked against: Python decimal: (620.9 − 631.9)/631.9; 60 × 620.9/631.9
In 225/45R17 94H, 225 is the tread width in mm, 45 is the aspect ratio (the sidewall is 45% of the width, 101.25 mm), R means radial construction and 17 is the rim diameter in inches. 94 is the load index, a maximum of 670 kg per tire, and H is the speed rating, 210 km/h. The calculator uses the size and ignores the load index and speed rating.
Add two sidewalls to the rim: diameter = rim × 25.4 + 2 × width × aspect ÷ 100, in mm. For 205/55R16 that is 406.4 + 2 × 112.75 = 631.9 mm, or 24.88 in. The circumference is π times the diameter, 1,985 mm here, and 1,000,000 divided by the circumference gives 503.7 revolutions per km.
Yes. The speedometer counts wheel turns, so a tire with a larger diameter covers more ground per turn than the speedometer assumes. True speed = shown speed × new diameter ÷ old diameter: going from 225/45R17 (634.3 mm) to 235/40R18 (645.2 mm), a reading of 100 means 101.7. A smaller tire does the opposite: from 205/55R16 to 195/55R16, a reading of 60 means 59.0.
Yes. UN Regulation 39, the vehicle approval rule for speedometers under the UNECE 1958 Agreement, requires that the indicated speed is never less than the true speed but allows it to be higher. Speedometers are therefore set to read at or above true speed, and fitting a taller tire, which lowers the reading, can use up that margin.
Divide 1,609,344 mm, one mile, by the circumference. A 225/45R17 tire has a diameter of 634.3 mm and a circumference of 1,992.7 mm, so it turns about 807.6 times per mile and 501.8 times per km. A taller tire turns fewer times, which is why the odometer also under-records distance after upsizing.
Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 4 worked examples whose answers come from independent sources; for example, “225/45R17 to 235/40R18” is checked against Python decimal: 17×25.4 + 2×225×0.45 = 634.3 mm; 18×25.4 + 2×235×0.40 = 645.2 mm; 10⁶/(π×634.3).
ETRTO — Standards Manual (metric tire designation: width / aspect ratio R rim); UNECE Regulation No. 39 — Speedometer and odometer (indicated speed never below true speed).
4 worked examples with independently sourced answers ship with this calculator. They run in the test suite; you can run them here too.
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