Tyre sizes explained: width, profile, wheel and rolling diameter
A tyre label such as 225/45 R17 contains enough information to estimate sidewall height, overall diameter and circumference. Those dimensions affect gearing and the relationship between wheel revolutions and road speed.
Read 225/45 R17 in three parts
Calculate overall diameter
For 225/45 R17: sidewall = 225 × 0.45 = 101.25 mm. The 17-inch wheel is 431.8 mm. Overall nominal diameter is therefore about 634.3 mm.
Example nominal dimensions
| Tyre size | Sidewall | Overall diameter | Circumference |
|---|---|---|---|
| 195/65 R15 | 126.8 mm | 634.5 mm | 1.993 m |
| 205/55 R16 | 112.8 mm | 631.9 mm | 1.985 m |
| 225/45 R17 | 101.2 mm | 634.3 mm | 1.993 m |
| 225/40 R18 | 90.0 mm | 637.2 mm | 2.002 m |
| 235/35 R19 | 82.2 mm | 647.1 mm | 2.033 m |
These are geometric nominal calculations, not a substitute for a tyre maker's measured specification. Real mounted dimensions can vary by tyre model, rim width, pressure, load and tread.
How a diameter change affects indicated speed
If the new tyre is larger, each wheel revolution travels farther. A simple geometric comparison is:
A 2% larger rolling diameter therefore implies roughly 2% more road distance per wheel revolution. Vehicle speedometers and control systems have their own calibration, so use this as a comparison rather than a claim about legal speedometer accuracy.
Fitment involves more than diameter
Width, wheel offset, rim width, load rating, speed rating, clearance, suspension travel and manufacturer-approved combinations all matter. A diameter calculator can show geometric change; it cannot certify that a wheel or tyre is safe or legal for a particular vehicle.
Compare percentage diameter change, not just millimetres
A 10 mm diameter change is proportionally larger on a small tyre than on a large one. The comparison used by MotoringCalcs is (new diameter ÷ original diameter − 1) × 100. This percentage gives a direct geometric indication of how rolling distance per revolution changes. It can also help flag when a proposed size is substantially different before you check the vehicle manufacturer's permitted fitments.