How to Read Tire Sizes: Numbers, Letters & Diameter
By AutomotiveCalc · Published September 9, 2026 · Method & standards
A code such as 225/45R17 combines millimeters, a percentage, and inches. Reading each part correctly is the first step to getting a useful tire calculation.
What does 225/45R17 mean?
- 225
- Nominal section width
225 millimeters - 45
- Sidewall height ÷ width
45 percent - R
- Construction
Radial - 17
- Wheel diameter
17 inches
The aspect ratio is a percentage, so 45 does not mean a 45 mm sidewall. R describes construction; it does not mean radius. A P prefix identifies a passenger tire and LT identifies a light-truck tire. Additional load and speed markings carry information beyond these dimensions. See Michelin’s tire-marking explanation.
In our Tire Size Calculator, paste the size portion into the optional entry box and choose Apply size. For 225/45R17, the editable fields become 225, 45, and 17. The shortcut also accepts P and LT prefixes; it does not assess the tire’s load capacity. Leave off a service description such as 91W. Flotation codes such as 35x12.5R17 use a different format and are not accepted by this shortcut.
Calculate the nominal tire diameter
Use the width and aspect ratio to find one sidewall’s height, then add two sidewalls to the wheel diameter. Convert the wheel measurement to millimeters before adding.
Diameter = wheel inches × 25.4 + 2 × sidewall
- Sidewall: 225 × 45 ÷ 100 = 101.25 mm.
- Wheel: 17 × 25.4 = 431.80 mm.
- Overall diameter: 431.80 + 2 × 101.25 = 634.30 mm.
- In inches: 634.30 ÷ 25.4 = 24.97 inches, rounded.
Open this 225/45R17 calculation →
The model treats the tire as an unloaded circle. Circumference equals π × diameter. Geometric revolutions per mile equal 63,360 divided by circumference in inches. These calculations describe nominal geometry, rather than a measured rolling test.
How changing a number changes the result
| Size | Sidewall (mm) | Diameter (in) |
|---|---|---|
| 225/45R17 | 101.25 | 24.97 |
| 235/45R17 | 105.75 | 25.33 |
| 225/40R18 | 90.00 | 25.09 |
Compare the first two rows: increasing width while keeping the same aspect ratio increases sidewall height too. Both tires have a 45% ratio, but 45% of 235 is greater than 45% of 225. In the third row, a larger wheel and a lower aspect ratio produce a diameter close to the first example. A larger wheel alone does not determine the tire’s overall diameter.
Compare 225/45R17 with 225/40R18 in the diagram →
The comparison tool divides new diameter by original diameter. If the original speedometer calibration was accurate, estimated actual speed equals indicated speed multiplied by that ratio. A ratio above one gives a higher estimated actual speed. This model does not account for a vehicle’s existing calibration error.
A similar diameter does not establish fitment
NHTSA recommends the original tire size or another size recommended by the vehicle manufacturer. Find the specified size in the owner’s manual or Tire and Loading Information Label, usually at the driver’s door. Its TireWise guidance also explains why the vehicle’s recommended inflation pressure should come from its label or manual, rather than the tire sidewall maximum.
These calculators do not contain a vehicle fitment database, wheel-width approvals, load or speed-rating checks, or clearance measurements. No percentage difference in their results is a pass/fail safety threshold. Confirm the complete specification for the vehicle before buying or fitting a replacement.
Sources and preparation
Michelin’s tire-marking guide and NHTSA TireWise were accessed September 9, 2026. The numerical examples above are AutomotiveCalc’s geometric calculations. Source links provide context and do not imply endorsement. Prepared with AI assistance; this guide has not received an independent automotive professional review. Read our editorial standards.