# Tire size and speedometer calculator

> Tire diameter, sidewall height and revolutions per km from a size such as 225/45R17, and the speedometer error when you change tire sizes.

Interactive version: https://www.calcopenly.com/everyday/tire-size-and-speedometer-calculator
Subject: Everyday calculators

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.

## Inputs

- **Current tire**
- **New tire**
- **Speedometer reading**
- **Speed unit** (options: km/h, mph)

## Results

- Diameter difference — main result
- True speed at that reading
- Current tire diameter (mm)
- New tire diameter (mm)
- Current tire diameter (in)
- New tire diameter (in)
- Current sidewall height (mm)
- New sidewall height (mm)
- Current tire revolutions (per km)
- New tire revolutions (per km)

## Formula

$$
\begin{gathered}D = \text{rim} \times 25.4 + 2 \times \text{width} \times \frac{\text{aspect}}{100}\ \text{mm} \\ v_{\text{true}} = v_{\text{shown}} \times \frac{D_{\text{new}}}{D_{\text{current}}}\end{gathered}
$$

## Worked examples

### 225/45R17 to 235/40R18

- Current tire: 225/45R17
- New tire: 235/40R18
- Speedometer reading: 100
- Speed unit: km/h
- **Current tire diameter: 634.3 mm**
- **New tire diameter: 645.2 mm**
- **Diameter difference: 1.72%**
- **True speed at that reading: 101.7 km/h**
- **Current sidewall height: 101.3 mm**
- **Current tire revolutions: 501.8 per km**
- 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)

### Same size

- Current tire: 205/55R16
- New tire: 205/55R16
- Speedometer reading: 100
- Speed unit: km/h
- **Diameter difference: 0.00%**
- **True speed at that reading: 100 km/h**
- Checked against: Identical diameters give no error

### Prefix, load index and speed rating

- Current tire: P215/60R16 94H
- New tire: 195/65R15
- Speedometer reading: 100
- Speed unit: km/h
- **Current tire diameter: 664.4 mm**
- **New tire diameter: 634.5 mm**
- **New tire revolutions: 501.7 per km**
- Checked against: Python decimal: 16×25.4 + 2×129 = 664.4; 15×25.4 + 2×126.75 = 634.5; 10⁶/(π×634.5)

### Smaller tire makes the speedometer over-read

- Current tire: 205/55R16
- New tire: 195/55R16
- Speedometer reading: 60
- Speed unit: km/h
- **Current tire diameter: 631.9 mm**
- **New tire diameter: 620.9 mm**
- **Diameter difference: -1.74%**
- **True speed at that reading: 59 km/h**
- Checked against: Python decimal: (620.9 − 631.9)/631.9; 60 × 620.9/631.9

## Questions

### What do the numbers on a tire mean?

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.

### How do you calculate tire diameter?

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.

### Does a bigger tire make the speedometer read slower?

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.

### Is a speedometer allowed to read higher than the true speed?

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.

### How many revolutions per mile does a tire make?

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.

### How accurate is the tire size and speedometer calculator?

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).

### Where does the method come from?

ETRTO — Standards Manual (metric tire designation: width / aspect ratio R rim); UNECE Regulation No. 39 — Speedometer and odometer (indicated speed never below true speed).

## Sources

- [ETRTO — Standards Manual (metric tire designation: width / aspect ratio R rim)](https://www.etrto.org/)
- [UNECE Regulation No. 39 — Speedometer and odometer (indicated speed never below true speed)](https://unece.org/transport/vehicle-regulations-wp29/standards/addenda-1958-agreement-regulations-21-40)