Tyre Size Calculator
Enter a tyre size and get its real dimensions. Add a second size and it will show how far out the speedometer would read.
Speedometer error
1.42%
- Overall diameter
- 634.3 mm
- In inches
- 24.97 in
- Sidewall height
- 101.3 mm
- Rolling circumference
- 1.993 m
- Revolutions per km
- 502
- New diameter
- 643.3 mm
- Difference
- 9 mm
- True speed at an indicated 100
- 101.4
- The larger tyre covers more ground per revolution, so the speedometer under-reads by 1.42%: an indicated 100 is really 101.4, and the odometer accumulates too slowly.
- Within about 3% is the usual guidance for staying legal and keeping the ABS and stability systems calibrated. This change is inside that.
- The notation mixes three unit systems. In 225/45R17 the 225 is millimetres of width, the 45 is the sidewall as a percentage of that width, and the 17 is the wheel in inches — so the same aspect number means a different height on a wider tyre.
- Check the load and speed ratings as well as the size. A tyre that fits is not necessarily one the vehicle is rated for.
About tyre sizes
A tyre size packs three different unit systems into five characters. In 225/45R17 the 225 is the width in millimetres, the 45 is the sidewall height as a percentage of that width, and the 17 is the wheel diameter in inches. Nobody would design it this way now; it is what a century of incremental standards produced.
The consequence that catches people out is that the aspect ratio is relative. A 45 series sidewall on a 225 tyre is 101mm; the same 45 on a 235 tyre is 106mm. Making the tyre wider makes it taller too, unless the aspect ratio comes down to compensate.
Overall diameter is what the car actually cares about. A speedometer counts wheel revolutions and multiplies by the diameter it was calibrated for, so fitting a larger tyre means each revolution covers more ground than the car believes. It reads low, and the odometer accumulates too slowly. A smaller tyre does the reverse.
Around three per cent is the usual guidance for how far out a replacement size can be before it matters. Beyond that the speedometer, the odometer, and the ABS and stability systems that infer speed from wheel sensors all start to be affected. Load and speed ratings are a separate check, and a tyre that physically fits is not necessarily one the vehicle is rated for.
What it works out
- Overall diameter, sidewall height and circumference
- Revolutions per kilometre
- Speedometer error when changing size
- True speed at an indicated 100
The formula
Diameter = (Width × Aspect ÷ 100) × 2 + (Wheel × 25.4)
Two sidewalls plus the wheel. The sidewall is the width times the aspect ratio, and the wheel is in inches so it needs converting at 25.4mm to the inch.
A 225/45R17 has a sidewall of 101.25mm. Two of those plus 431.8mm of wheel gives an overall diameter of 634.3mm, or just under 25 inches. Its rolling circumference is 1.993 metres, so it turns about 502 times per kilometre.
Move to a 235/45R17 and the sidewall grows with the width: 45% of 235 is 105.75mm, so the diameter becomes 643.3mm. That is 9mm larger, or 1.42%.
The speedometer error is that same percentage. Each revolution now covers 1.42% more ground than the car assumes, so at an indicated 100 you are really doing 101.4. Under three per cent, this is within the range normally considered acceptable.
- Width
- Section width in millimetres — the widest point of the tyre, not the tread.
- Aspect
- Sidewall height as a percentage of the width, so it means a different height on a different width.
- Wheel
- Rim diameter in inches, which is why the 25.4 appears.
A worked example
Replacing a 225/45R17 with a 235/45R17.
That works out to 1.42 %.
- Overall diameter
- 634.3 mm
- In inches
- 24.97 in
- Sidewall height
- 101.3 mm
- Rolling circumference
- 1.993 m
- Revolutions per km
- 502
- New diameter
- 643.3 mm
- Difference
- 9 mm
- True speed at an indicated 100
- 101.4
Questions
What do the numbers on a tyre mean?
In 225/45R17: 225 is the width in millimetres, 45 is the sidewall as a percentage of that width, R means radial construction, and 17 is the wheel diameter in inches.
How much can I change tyre size by?
About three per cent in overall diameter is the usual guidance. Beyond that the speedometer and odometer drift noticeably and the ABS and stability systems, which infer speed from wheel rotation, are working from bad data.
Does a bigger tyre make the speedometer read low or high?
Low. Each revolution covers more ground than the car was calibrated for, so you are going faster than it says. A 1.42% larger tyre means an indicated 100 is really 101.4, and the odometer counts too few miles.
Will a wider tyre fit my existing wheels?
Sometimes — each rim width takes a range of tyre widths, usually spanning about 20mm. Going outside that range distorts the tyre's profile and affects both handling and wear, so check the rim width against the tyre manufacturer's table.
Does a wider tyre also make the car taller?
Yes, if the aspect ratio stays the same, because the aspect ratio is a percentage of the width. Going from 225 to 235 at a 45 series adds about 4.5mm of sidewall, and 9mm of overall diameter.
Why is my speedometer optimistic from the factory?
Regulations in most countries require a speedometer never to under-read, so manufacturers calibrate them to read slightly high — often two to five per cent. A slightly larger tyre eats into that margin rather than immediately making the car read low.
What is rolling circumference used for?
It is what converts wheel revolutions into distance, so it underpins the speedometer, the odometer and the traction systems. It is also why all four tyres should be close in diameter, particularly on four-wheel drive.
Do load and speed ratings matter?
Yes, and they are a separate check from size. The load index and speed symbol must meet what the vehicle requires — a tyre that fits the wheel is not necessarily rated for the car.