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
| Field | What to enter |
|---|---|
| Current tyre width | In mm |
| Current aspect ratio | In % |
| Current rim diameter | In in |
| New tyre width | In mm |
| New aspect ratio | In % |
| New rim diameter | In in |
| Speedometer reading | In 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) |
|---|---|
| 112.5 | 533.1 |
| 168.75 | 583.7 |
| 225 | 634.3 |
| 281.25 | 684.9 |
| 337.5 | 735.6 |
| 450 | 836.8 |
Formula
Sidewall = width × aspect ratio ÷ 100Diameter = rim + 2 × sidewallRevolutions per mile = 63,360 ÷ (π × diameter in inches)Actual speed = indicated speed × new diameter ÷ old diameterWorked 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.
Frequently asked questions
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.