How to read a tyre size, load index and sidewall markings

The 225 is nominal section width in millimetres, measured under specified conditions. Section width describes the tyre across its sidewalls, not simply the width of the tread touching the road. The 45 is the aspect ratio: nominal sidewall height is 45% of that section width. R describes radial construction. The 17 is the wheel's bead-seat diameter in inches, not its radius.

After the size you will normally find a load index and speed symbol, such as 94W. These are codes with defined meanings, not a weight of 94 kg or a speed of 94 mph. Read them against the tyre manufacturer's tables and the vehicle's requirements. Other markings can identify reinforced construction or a tyre developed for a particular vehicle application.

For 225/45 R17, nominal sidewall height is 225 × 0.45 = 101.25 mm. The rim diameter is 17 × 25.4 = 431.8 mm. Add a sidewall above and below the rim: 431.8 + 101.25 + 101.25 = 634.3 mm. This is the nominal unloaded diameter used in basic comparison tools.

That calculation explains why the same aspect ratio produces different sidewall heights on different widths. A 245/45 tyre has a nominal sidewall of 110.25 mm, 9 mm taller than a 225/45 tyre. On the same rim diameter, the complete tyre is 18 mm larger in diameter. Keeping the number 45 does not preserve tyre height when width changes.

Circumference is approximately diameter multiplied by pi. For our 634.3 mm example, that is about 1,992.7 mm. A real tyre's rolling circumference differs because load, pressure, tread wear and construction affect its shape. Use the manufacturer's published dimensions and vehicle compatibility guidance when selecting a final replacement.

• Compare two complete tyre sizes

The complete replacement specification has to support the vehicle's loads and intended use. Do not choose a tyre purely because its dimensions match a calculator result. Ask the tyre supplier to confirm the load and speed requirements for your exact trim, including any different front and rear specifications. The tyre placard and handbook are your starting point.

XL indicates extra-load construction within the relevant tyre category. It does not mean that every XL tyre carries more weight than every non-XL tyre in another size, and it does not tell you the correct inflation pressure. Read the load index and the maker's load-pressure information alongside it. Larger wheels sometimes require lower-profile replacements whose load capability needs particularly careful checking.

Toyo's safety guidance calls for approved rim widths and adequate wheel load and inflation capacity when changing sizes. That applies even when the tyre can physically be mounted. A fitter should assess the tyre, rim and vehicle as an assembly, rather than treating each label as independent permission.

Start with the vehicle maker's cold-pressure specification for the correct approved size and load condition. The maximum value moulded on a tyre is not automatically the everyday pressure for your car. A front axle and rear axle may need different values, and the handbook may provide a separate setting for a fully loaded journey.

If you change to a size outside the listed options, ask a qualified tyre professional to establish a suitable pressure from the applicable load information. Copying another owner's number is unreliable if their tyre construction, axle load or use differs. Pressure should not be used to compensate for a tyre mounted outside its approved rim range.

Keep a record of whether a pressure was measured cold or after driving. If you are comparing ride quality after a wheel change, differing temperature or pressure can confuse the result. A sensible comparison starts with verified cold settings and the same normal load, then checks that the tyre remains appropriate for the car.

Look for directional arrows and any inside or outside designation, because these affect installation and rotation. A tyre can have an asymmetric tread without being directional. If it has both requirements, both must be respected. Ask the fitter to confirm the orientation on every wheel before the car leaves.

Check the complete product name as well as size. Two tyres with identical numbers can have different shoulder shapes, measured widths and intended applications. Vehicle-specific versions can also exist under the same broad model name. A seller's short listing title may omit that detail, so compare the full manufacturer product code when matching an existing set.

• Read all four fitted tyres rather than assuming they match.
• Compare the markings with the placard and handbook for your trim.
• Record tyre model, size, load index, speed symbol and any vehicle-specific designation.
• Check the maker's approved rim widths and actual dimensions.
• Resolve any missing or conflicting information with the supplier before ordering.

Imagine your existing tyre is 225/45 R17 and a shop offers a wider 245/45 R17. The rim diameter matches, but the nominal diameter rises from 634.3 to 652.3 mm, about 2.84%. Width also increases. You now have separate questions about rolling size, arch clearance, rim approval and ratings. The shared R17 marking only answers the rim-diameter question.

The most useful order of work is to identify the vehicle's requirements, choose an appropriate tyre model and size, then verify its relationship to the actual wheel. Save the specification sheet with your order. If a tyre is substituted because of stock availability, repeat the relevant checks instead of assuming every product in the same nominal size is identical.

Finally, inspect the delivered sidewall before fitting. A single wrong aspect ratio or load variant can be easy to miss in a stack of four tyres. Catching a mismatch while the tyres are still unmounted makes correction simpler and avoids turning a paperwork mistake into a road problem.