# Tire Size Calculator

URL: https://speedycalculator.com/calculators/everyday-life/tire-size/
Last updated: 2026-10-25

> The tire size calculator decodes a tyre size like 225/45R17 into sidewall height, overall diameter, circumference and revolutions per mile, compares two sizes and shows how a change affects your speedometer.

## Inputs

- Current tyre width (mm)
- Current aspect ratio (%)
- Current rim diameter (in)
- New tyre width (mm)
- New aspect ratio (%)
- New rim diameter (in)
- Speedometer reading (mph or km/h)

## Results

- Current sidewall height
- Current overall diameter
- Current circumference
- Current revolutions per mile
- New overall diameter
- Diameter difference
- Actual speed on new tyres

## Formula

```
Sidewall = width × aspect ratio ÷ 100
Diameter = rim + 2 × sidewall
Revolutions per mile = 63,360 ÷ (π × diameter in inches)
Actual speed = indicated speed × new diameter ÷ old diameter
```

## Explanation

A metric tyre size gives the section width in millimetres, the aspect ratio (sidewall height as a percentage of width) and the rim diameter in inches. Changing to a tyre with a different overall diameter makes the speedometer read wrong – most guides recommend staying within ±3%.

### How to use the Tire Size Calculator

- **Current tyre width**
- **Current aspect ratio**
- **Current rim diameter**
- **New tyre width**
- **New aspect ratio**
- **New rim diameter**
- **Speedometer reading**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Enter the width, aspect ratio and rim diameter of the current tyre and the new tyre, and a speedometer reading to see your true speed on the new tyres.

### Tips

- Use your own measured figures – fuel economy, speed and prices vary a lot with driving style, traffic, load and season.
- Pick the unit in each field's menu (miles or kilometres, mpg or L/100 km); the calculator converts everything for you.
- Fill up to the brim twice and divide the distance by the fuel added to measure real fuel economy.
- For vehicle modifications, treat results as a starting point and confirm with the parts maker or a professional mechanic.

### Common mistakes to avoid

- Mixing US and imperial (UK) gallons – a UK gallon is about 20% bigger.
- Using the advertised fuel economy instead of real-world figures.
- Forgetting tolls, parking and wear and tear when comparing driving with public transport.

### Inputs explained

FieldWhat to enterCurrent tyre widthIn mmCurrent aspect ratioIn %Current rim diameterIn inNew tyre widthIn mmNew aspect ratioIn %New rim diameterIn inSpeedometer readingIn mph or km/h

### What each result means

- **Current sidewall height** = [Current tyre width]×[Current aspect ratio]÷100
- **Current overall diameter** = [Current rim diameter]×25.4 + 2×[Current sidewall height]
- **Current circumference** = pi×[Current overall diameter]
- **Current revolutions per mile** = 1609344÷[Current circumference]
- **New overall diameter** = [New rim diameter]×25.4 + 2×[New tyre width]×[New aspect ratio]÷100
- **Diameter difference** = ([New overall diameter]÷[Current overall diameter] - 1)×100
- **Actual speed on new tyres** = [Speedometer reading]×[New overall diameter]÷[Current overall diameter]

### Example values

How the main result changes when **Current tyre width** changes and everything else stays at the example values above:

Current tyre width (mm)Current overall diameter (mm)112.5533.1168.75583.7225634.3281.25684.9337.5735.6450836.8

## Worked example

Inputs — Current tyre width: 225 mm; Current aspect ratio: 45 %; Current rim diameter: 17 in; New tyre width: 235 mm; New aspect ratio: 45 %; New rim diameter: 17 in; Speedometer reading: 60 mph or km/h.

Results — Current sidewall height = 101.3 mm; **Current overall diameter = 634.3 mm**; Current circumference = 1,993 mm; Current revolutions per mile = 808; New overall diameter = 643.3 mm; Diameter difference = 1.42 %; Actual speed on new tyres = 60.9.

## Key terms

- **Aspect ratio**: Sidewall height as a percentage of the tyre's section width.

## FAQ

**What does 225/45R17 mean?**
225 mm section width, sidewall 45% of the width (101 mm), radial construction, 17-inch rim.

**How much can tyre diameter change?**
Most guides recommend within ±3% of the original to keep the speedometer, ABS and gearing close to factory.

**Will bigger tyres make my speedometer read low?**
Yes – with larger tyres you travel faster than the speedometer shows.

**What formula does the Tire Size Calculator use?**
Sidewall = width × aspect ratio ÷ 100.