Tire size comparison explained
A metric tire size gives nominal section width, sidewall aspect ratio, construction, and wheel diameter. Those values estimate overall diameter but do not guarantee vehicle fitment.
225/65R17 vs. 235/60R18
28.52 in original diameter vs. 29.10 in new diameter
Decode 225/65R17
| Marking | Meaning |
|---|---|
| 225 | Nominal section width in millimeters |
| 65 | Sidewall height is 65% of section width |
| R | Radial construction |
| 17 | Wheel diameter in inches |
The diameter calculation
Estimated circumference changes by the same percentage as diameter, so indicated road speed changes by approximately 2.06% in this example.
Diameter is only one fitment check
Replacement tires must also match the vehicle’s requirements for load capacity, speed rating, construction, and approved wheel dimensions. Wheel width, offset, brake and suspension clearance, fender clearance, chains, driver-assistance systems, and spare-tire compatibility can matter even when diameter is close.
What the calculator can estimate
- Nominal sidewall height.
- Overall unloaded diameter.
- Circumference and approximate revolutions per mile.
- Percentage difference between sizes.
- Estimated actual speed at a chosen indicated speed.
Real rolling circumference varies with load, inflation, wear, rim width, and the exact tire model.
Frequently asked questions
Why does a taller tire change speedometer readings?
Each revolution travels farther, so the vehicle covers more distance than expected from the original calibration.
Does R17 mean a 17-inch tire?
It means the tire fits a 17-inch wheel. The full tire diameter includes two sidewalls in addition to the wheel diameter.
Is a 2% difference always safe?
No universal percentage guarantees fitment or system compatibility. Use the calculation as a comparison and follow vehicle-specific guidance.