What does 225/45R17 mean?

225 is the section (tread) width in millimetres. 45 is the aspect ratio — the sidewall height is 45 percent of the width. R means radial construction, and 17 is the wheel rim diameter in inches. Together they fix the tyre's physical size.

What is the aspect ratio?

The aspect ratio is the sidewall height expressed as a percentage of the section width. A 45 series tyre 225 mm wide has a sidewall of 225 × 0.45 = 101 mm. Lower numbers mean shorter, sportier sidewalls; higher numbers mean taller, more comfortable ones.

What do the load index and speed rating mean?

The two-digit load index maps to a maximum load per tyre (for example 94 = 670 kg), and the letter speed rating maps to a top speed (W = 270 km/h). Both must meet or exceed your vehicle's requirements; never fit a lower-rated tyre.

How is overall diameter calculated?

Overall diameter equals the rim diameter (converted to mm) plus twice the sidewall height. For 225/45R17: rim 17 in = 431.8 mm, sidewall 101.25 mm, so diameter = 431.8 + 2 × 101.25 ≈ 634 mm. Keeping diameter within ~3 percent of original preserves speedometer accuracy.

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Tyre Size Designation Reference

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ISO metric tyre sizes

The cryptic code on a tyre sidewall, such as 225/45R17 94W, fully describes its size, construction and ratings under the ISO metric system. Reading it correctly matters for fitment, safety and speedometer accuracy. This tool decodes any standard code into its parts and computes the sidewall height and overall diameter.

How it works

The code parses as width / aspect R rim load-index speed-rating. The section width is in millimetres; the aspect ratio is the sidewall height as a percentage of that width, so sidewall = width × aspect ÷ 100. The letter (usually R for radial) is the construction. The rim diameter is in inches, converted at 25.4 mm/in. Overall diameter = rim(mm) + 2 × sidewall. The two-digit load index and trailing letter map through standard tables to a maximum load in kilograms and a top speed in km/h.

Tips and examples

When changing tyre size, keep overall diameter within about 3% of the original or your speedometer and odometer will read wrong and ABS/traction systems may misbehave. A lower aspect ratio (e.g. 40 vs 55) sharpens steering response but transmits more road shock. Always match or exceed the vehicle’s specified load index and speed rating — fitting a 91H tyre where 94W is required is both unsafe and, in many countries, illegal. The size on the door placard, not the worn tyre already fitted, is the authority.