Results are dimensional estimates. Actual mounted dimensions vary by manufacturer and rim width.
How to compare two tire sizes
A tire-size comparison translates the sidewall code into theoretical dimensions. Enter the current and proposed sizes to compare sidewall height, overall diameter, radius, circumference and revolutions per mile or kilometre. The calculation helps you understand scale, but it does not confirm that the replacement is approved for your vehicle or wheel.
What the three tire-size numbers mean
In 225/45R17, 225 is nominal section width in millimetres, 45 is the sidewall height as a percentage of that width, and 17 is the wheel diameter in inches. The letter R describes radial construction and is not a numeric input. Use the complete size from the sidewall or vehicle placard.
Formula and assumptions
Sidewall height equals width multiplied by aspect ratio divided by 100. Rim diameter in millimetres equals rim diameter in inches multiplied by 25.4. Overall diameter equals rim diameter plus two sidewalls. Circumference equals diameter multiplied by pi. Revolutions per distance are calculated by dividing that distance by circumference.
Worked example
Compare 205/55R16 with 225/45R17. Although the second tire is wider and uses a larger wheel, its lower-profile sidewall keeps the overall diameter close to the first size. Review the percentage difference, not only the wheel diameter, and then check whether the wider tire is approved for the proposed rim and has adequate vehicle clearance.
How to interpret the comparison
- Sidewall height: theoretical distance from bead seat to outer tread radius.
- Overall diameter: theoretical unloaded diameter of the tire.
- Circumference: theoretical distance travelled in one full revolution.
- Revolutions: estimated rotations needed to travel one mile or kilometre.
- Percentage difference: relative change between the two calculated diameters.
Common uses and safety checks
Use this tool to screen plus-size or downsized options, compare a seasonal tire, examine sidewall changes, or estimate which sizes have similar nominal diameters. Then confirm the vehicle manufacturer specification, approved rim-width range, load index, speed rating, pressure requirement and available clearance. Actual mounted dimensions vary by tire model and rim width.
Frequently asked questions
What percentage diameter difference is acceptable?
There is no universal safe percentage. Vehicle manufacturers, local rules and electronic systems differ, so treat the percentage as comparison data and verify the specific application.
Why does a wider tire not always have a larger diameter?
Aspect ratio is a percentage of width and wheel diameter also changes. All three numeric parts determine overall diameter.
Does this calculate tread width?
No. The width code is nominal section width, and actual section and tread widths vary by product and measuring rim.
Sources and further checks
- NHTSA tire-safety guidance
- Your vehicle owner manual and tire-information placard
- The wheel and tire manufacturers’ current technical specifications