About this calculator
The Tyre Size Calculator converts a tyre code such as 205/55 R16 into physical dimensions. It estimates sidewall height, overall diameter, circumference, and the difference between an original and alternative size. It is useful when checking replacement tyres, wheel changes, speedometer effect, and whether a size change is broadly comparable.
Tyre Size Calculator method
This calculator uses straightforward fuel economy, vehicle measurement, or unit conversion formulas. Results are estimates for planning and comparison, so real-world driving, vehicle condition, load, weather, route type, and manufacturer data can change the outcome.
- sidewall height mm = section width x aspect ratio / 100
- overall diameter mm = rim inches x 25.4 + 2 x sidewall height
- circumference = pi x overall diameter
- speed difference percent = new circumference / old circumference - 1
How to use the Tyre Size Calculator
- Enter the main vehicle or journey figures requested by the calculator.
- Use UK units where shown, especially miles, litres, UK MPG, and prices per litre.
- Check whether the calculator is asking for measured real-world figures or official brochure figures.
- Review the headline result and the supporting breakdown below it.
- Compare at least two scenarios if you are choosing between vehicles, journeys, or running-cost assumptions.
- Adjust one assumption at a time to see which input changes the result most.
- Use the result as an estimate, then confirm important decisions with real quotes, vehicle records, or specialist advice.
Worked examples
Common family car tyre
Input: 205/55 R16
Calculation: Sidewall 205 x 0.55 = 112.75mm; rim 16 x 25.4 = 406.4mm
Result: Overall diameter about 631.9mm
Compare larger wheel
Input: Original 205/55 R16, new 225/45 R17
Calculation: Compare total circumference for both sizes
Result: The calculator shows the percentage speedometer difference
What the tyre code means
- Width
- The first number is tyre section width in millimetres.
- Aspect ratio
- The second number is sidewall height as a percentage of width.
- Rim size
- The final number after R is wheel rim diameter in inches.
Why tyre size affects speed
A larger rolling circumference travels farther per wheel rotation. That can change indicated speed, gearing feel, acceleration, ride comfort, and clearance. Small differences are common, but large changes should be checked carefully.
Why real-world results vary
Fuel economy and running costs can shift noticeably from the calculated result. Short trips, cold starts, roof boxes, tyre pressure, motorway speed, stop-start traffic, heavy loads, air conditioning, and poor servicing can all increase fuel use.
For buying decisions, compare a conservative scenario as well as an optimistic one. A car that looks cheaper on official MPG may cost more if your real-world driving pattern is mainly urban or short-distance.
Common mistakes and edge cases
- Tyre width does not equal tread width exactly.
- Actual tyre dimensions vary by manufacturer and model.
- Load index, speed rating, clearance, insurance, and vehicle approval still matter.
- Run-flat, winter, commercial, and specialist tyres may behave differently.
Limitations
This calculator is for general information only. It is not financial, engineering, insurance, valuation, or mechanical advice.
- This is a geometry estimate only and does not confirm legal or safe fitment.
- It does not check wheel width, offset, arch clearance, brake clearance, or insurance requirements.
- Ask a qualified tyre fitter before changing away from approved sizes.
Frequently asked questions
What does 205/55 R16 mean?
It means 205mm width, sidewall height 55% of width, and a 16-inch rim.
Can I fit any equivalent diameter tyre?
No. Width, load rating, speed rating, clearance, wheel size, and approval all matter.
Why does circumference matter?
It affects how far the car travels per wheel rotation and can change speedometer reading.
Is a small speed difference acceptable?
Small differences are common, but check manufacturer and tyre professional guidance.
Does this calculate tyre pressure?
No. Use the vehicle placard or handbook for tyre pressure.
Related calculators
- MPG Calculator
- Fuel Cost Calculator
- Car Depreciation Calculator