How to Read Tire Size: Width, Aspect Ratio and Rim Diameter

Read 225/45R17 one part at a time

The size on a tire sidewall is a compact specification, rather than a single measurement. In 225/45R17, 225 describes nominal section width in millimeters, 45 describes sidewall height as a percentage of that width, R identifies radial construction, and 17 identifies the matching rim diameter in inches. The mixture of units is normal. Converting every number directly to inches would lose the meaning of the aspect ratio.

Start by copying the complete marking from the existing tire, then compare it with the vehicle tire placard or owner’s manual. A tire currently installed on a used vehicle may already differ from the original specification. Record front and rear sizes separately when the vehicle uses different sizes on the two axles.

Turn the marking into nominal dimensions

For 225/45R17, one sidewall is 225 × 0.45 = 101.25 mm tall in the nominal model. The 17-inch rim contributes 17 × 25.4 = 431.8 mm. Adding two sidewalls gives a nominal outside diameter of 431.8 + 101.25 + 101.25 = 634.3 mm, or approximately 24.97 inches. The tire size calculator performs these conversions and also reports theoretical circumference.

The 17-inch number is therefore neither the tire’s outside diameter nor its sidewall height. It describes the rim that the tire is designed to mount on. A tire marked R17 cannot be used as an R18 tire because its calculated outside diameter happens to be close. Each tire and rim pair must have the correct matching diameter and approved specifications.

Why the same aspect ratio can give different heights

Aspect ratio depends on width. A 225/45 tire has a nominal 101.25 mm sidewall, while a 245/45 tire has a nominal 110.25 mm sidewall. The difference is 9 mm on each side of the rim, even though both markings contain 45. Holding rim diameter constant, the second tire is consequently 18 mm taller in the nominal calculation.

This is useful when reading a tire comparison: changing width can also change diameter. Compare complete sizes instead of judging only the first number or assuming that all 45-series tires have one sidewall height. The same reasoning explains why a larger wheel often uses a lower aspect ratio when the goal is to keep overall tire diameter close to a reference size.

Keep load information and real dimensions separate

Additional markings such as P, LT, a load index and a speed symbol describe matters beyond the geometric size. For example, 91H following a size is a service description; it is not another dimension to feed into the diameter formula. Copy that information into your replacement notes rather than discarding it after decoding the size.

Section width is also different from tread width and from maximum overall width. Protective sidewall ribs may extend beyond the section measurement. Yokohama defines outside diameter on a measuring rim without a vehicle load. Two tire models with the same size marking can therefore have different published dimensions. For tight clearance work, obtain the exact product specification and its measuring-rim information.

Use the marking to understand and compare nominal geometry, then use the vehicle requirements and the exact tire data to assess a replacement. This keeps a useful size calculation from becoming an unsupported fitment recommendation.

Common questions