Tire diameter, sidewall height and revolutions per km from a size such as 225/45R17, and the speedometer error when you change tire sizes.
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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 section 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.
A metric tire's overall diameter is rim × 25.4 + 2 × width × aspect ÷ 100, in millimeters. A 225/45R17 is 634.3 mm tall and a 235/40R18 is 645.2 mm, so the new size is 1.72% taller. With the speedometer showing 100, the car is then doing about 101.7.
Take 225/45R17 94H:
For 225/45R17:
One kilometer is 1,000,000 mm, so the tire turns 1,000,000 ÷ 1,992.7 = 501.8 times per km, or 807.6 times per mile.
The 235/40R18 has a 457.2 mm rim and two 94 mm sidewalls, giving 645.2 mm. The rim grew by an inch, yet each sidewall lost 7.25 mm, so the whole tire grew only 10.9 mm. Compare diameters, not rim sizes.
A speedometer counts wheel turns, so a taller tire covers more ground per turn than it was calibrated for. True speed = indicated speed × new diameter ÷ old diameter.
For the pair above, the ratio is 645.2 ÷ 634.3 = 1.0172. An indicated 100 km/h is 101.7 km/h, and an indicated 60 mph is 61.0 mph: the same multiplier applies in either unit.
A smaller tire does the opposite. Going from 205/55R16 (631.9 mm) to 195/55R16 (620.9 mm) is 1.74% shorter, so an indicated 60 mph is really 59.0 mph.
The odometer is off by the same ratio. After the switch to 235/40R18, 1,000 km on the odometer is 1,017.2 km driven, which also makes fuel-economy figures read about 1.7% low.
UN Regulation 39 requires a speedometer never to show less than the true speed, so most read slightly high from the factory. A taller tire uses up part of that margin.
| Scenario | Current diameter, mm | New diameter, mm | Difference, % |
|---|---|---|---|
| Default | 634.300000 | 645.200000 | 1.718430 |
| Same size | 631.900000 | 631.900000 | 0.000000 |
| Smaller tire | 631.900000 | 620.900000 | -1.740782 |
| Prefixes and ratings | 664.400000 | 634.500000 | -4.500301 |
| Decimal rim | 810.300000 | 838.300000 | 3.455510 |
P215/60R16 94H to 195/65R15. The prefix, load index and speed rating do not affect the diameter.LT225/70R19.5 128/126M to ST245/70R19.5, half-inch rims used on trucks and trailers.The percentage is always relative to the current tire. Comparing 235/40R18 back to 225/45R17 gives −1.69%, not −1.72%, because the base becomes 645.2 mm.
225/45R17, with an optional P, LT, T or ST prefix and an optional load index and speed rating.33x12.50R15 use a different format and are not read.To see how the odometer error affects a fuel log, run the corrected distance through the gas mileage calculator.
Checked against: 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: 16×25.4 + 2×129 = 664.4; 15×25.4 + 2×126.75 = 634.5; 10⁶/(π×634.5)
Checked against: (620.9 − 631.9)/631.9; 60 × 620.9/631.9
In 225/45R17 94H, 225 is the section 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.
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