How to use the tire size calculator
- Enter the size printed on the sidewall of your current tire as Size A, for example
205/55 R16. You can type the whole marking or fill in width, aspect ratio and rim separately. - To compare, enter the size you are considering as Size B.
- Optionally enter a speed to see the theoretical road speed when the speedometer shows that figure with Size B fitted.
- Press Calculate. Results update automatically when you change an input afterwards.
In a marking such as 205/55 R16, 205 is the nominal section width in millimetres, 55 is the aspect ratio (sidewall height as a percentage of width), R means radial construction and 16 is the rim diameter in inches. Any load index and speed symbol after the size, such as 91V, is accepted and ignored because it does not affect dimensions.
Worked example: 205/55 R16 to 225/45 R17
A common “plus-one” change moves from a 16-inch to a 17-inch wheel with a lower-profile tire.
| Measurement | A: 205/55 R16 | B: 225/45 R17 | Difference |
|---|---|---|---|
| Sidewall height | 112.8 mm | 101.3 mm | −11.5 mm |
| Overall diameter | 631.9 mm | 634.3 mm | +2.4 mm (+0.38%) |
| Circumference | 1,985 mm | 1,993 mm | +7.5 mm |
| Revolutions per mile | 810.7 | 807.6 | −3.1 |
Size B is 2.4 mm larger in diameter, so each revolution covers slightly more road. If the speedometer was calibrated for Size A and reads 100, the theoretical road speed with Size B is about 100.4. The nominal ride height would rise by about 1.2 mm.
What the results mean
Diameter and circumference
The overall diameter is the rim diameter plus two sidewalls. Circumference is that diameter times π, so the percentage change in circumference is the same as the percentage change in diameter.
Speedometer difference
A speedometer effectively counts wheel revolutions and assumes each one covers the circumference of the size it was set up for. If Size B is larger, every revolution covers more ground, so the true road speed is higher than the speedometer shows. If Size B is smaller, the true speed is lower. The odometer is affected in the same proportion.
Ride height
The axle centre sits roughly one radius above the road, so a change in radius changes ride height by about the same amount. This ignores tire deflection under load.
Not a fitment check. A dimensional comparison does not confirm that a size fits or is approved. Clearance, load index, speed rating, rim width, electronic systems such as ABS and stability control, and local regulations all matter.
Formulas used
sidewall (mm) = width (mm) × aspect ratio ÷ 100
overall diameter (mm) = rim (in) × 25.4 + 2 × sidewall
circumference (mm) = π × overall diameter
revolutions per km = 1,000,000 ÷ circumference (mm)
revolutions per mile = 1,609,344 ÷ circumference (mm)
true speed = indicated speed × circumference B ÷ circumference A One inch is exactly 25.4 mm and one mile is exactly 1,609.344 m. Calculations use full precision; values are only rounded for display.
Sources: the size designation follows the metric passenger-car system described in ISO 4000-1 (Passenger car tyres and rims) and explained for consumers by the Tire Industry Association.
Limitations
- All values are nominal geometric estimates from the size marking, not measurements of a particular tire.
- Real dimensions vary between brands and models; rolling circumference also changes with tread wear, pressure, load, temperature and speed.
- The speed comparison assumes the speedometer was exactly calibrated for Size A.
- Only metric passenger-car notation is supported.
Frequently asked questions
Is “tire” or “tyre” correct?
Both. “Tire” is the usual spelling in the United States and Canada; “tyre” is used in the UK, Australia and most other English-speaking markets. They mean the same thing and the size markings are identical.
Which size is the speedometer assumed to be calibrated for?
This calculator assumes the speedometer was calibrated for Size A. The comparison then shows what the theoretical true road speed would be with Size B fitted when the speedometer shows your chosen speed. Real speedometers are not perfectly calibrated even on the original size, so treat the result as the geometric change only.
Does a small percentage difference mean a size is safe to fit?
No. This tool compares nominal dimensions only. Whether a size can be fitted depends on load index, speed rating, rim width range, clearance through full steering and suspension travel, vehicle electronics and local rules. Check the vehicle manufacturer’s approved sizes or ask a qualified tyre professional.
Why don’t my measured dimensions match these numbers?
Sizes are nominal. Each tyre model has its own construction and tread depth, and the loaded rolling radius is smaller than the free radius. Wear, pressure, load, temperature and speed all change the effective circumference. Manufacturers often publish revolutions per mile or per kilometre, which is a better figure for a specific tyre.
Which tyre markings are supported?
Metric passenger-car sizes in the form width/aspect R rim, such as 205/55 R16, P215/65R15, 225/40ZR18 92Y, run-flat “RF” and commercial “C” suffixes. Light-truck (LT), flotation (31x10.50R15), millimetric-rim, bias-ply and sizes without an aspect ratio (185R14) are not supported, and the calculator says so instead of guessing.
How is ride height change estimated?
As the difference in nominal radius: half of the overall diameter difference. Actual ride height also depends on tyre deflection under load and the suspension, so this is an approximation of how much the axle centre would rise or fall.
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