Every tyre on your car carries a short string of numbers and letters on its sidewall, and almost nobody reads it. It looks like a code, and in a way it is — but once you know what each part means, you can tell at a glance whether the tyres fitted to your car are actually the right ones for it.
Here is how to read it, using a common example: 225/45R17 94W.
Breaking down the numbers
225 — section width
This is the width of the tyre in millimetres, measured across the widest part of the sidewall when the tyre is fitted and inflated. A wider tyre generally offers more grip, but it also costs a little more fuel and can be noisier.
45 — aspect ratio
This is the sidewall height expressed as a percentage of the width. So a 45 here means the sidewall is 45% of 225mm, or roughly 101mm tall. Lower numbers mean shorter, stiffer sidewalls — sharper steering response, but a firmer ride and less protection from potholes and kerbs.
R — construction
R stands for radial, which is what almost every modern passenger car tyre uses. You may occasionally see a D (diagonal or bias-ply) on trailers and older vehicles.
17 — rim diameter
The wheel size in inches that this tyre is built to fit. A 17 here will only go on a 17-inch rim. This one is non-negotiable.
Load index and speed rating
The last two characters — 94W in our example — are the ones most often overlooked, and they matter more than people realise.
The load index (94) is a coded figure that corresponds to the maximum weight that single tyre can safely carry when correctly inflated. It is not kilograms directly; it maps to a standard table. An index of 94, for instance, corresponds to roughly 670kg per tyre. Fitting tyres with a lower load index than your vehicle requires means the tyre is being asked to do more than it was built for — a real concern for utes, 4WDs, vans and anyone who tows or carries heavy loads regularly.
The speed rating (W) is a letter indicating the maximum speed the tyre is designed to sustain. Common ratings include:
- T — 190km/h
- H — 210km/h
- V — 240km/h
- W — 270km/h
- Y — 300km/h
Australian speed limits sit well below all of these, so the point is not that you will ever drive that fast. The speed rating reflects how the tyre is constructed — its heat tolerance, its stability under load, how it behaves during hard braking and emergency manoeuvres. A higher-rated tyre is built to hold its shape under more stress.
Why it matters for your car specifically
Your vehicle was designed around a particular tyre specification. You will find it on the tyre placard — usually a sticker inside the driver's door jamb, sometimes in the glovebox or fuel filler flap — along with the recommended inflation pressures.
Fitting tyres below the placard's load index or speed rating can affect handling, and in some cases can create problems with your insurance or a roadworthy inspection. It is worth a thirty-second check next time you are standing next to your car: compare what is on the placard with what is actually stamped on the sidewall. If someone fitted a cheaper set at some point, the mismatch will show up here.
The other markings worth knowing
A few more things you will find on that sidewall:
- DOT code — a four-digit number showing the week and year of manufacture. "3224" means week 32 of 2024. Rubber ages whether you drive on it or not.
- Tread wear indicators — small raised bars in the tread grooves, often marked by a triangle or "TWI" on the sidewall. When the tread is level with these bars, you are at the 1.5mm legal minimum.
- Rotation arrows — some tyres are directional and must be fitted to turn a particular way.
- Max pressure — this is the tyre's structural limit, not your recommended pressure. Always use the placard figure instead.
If you are unsure whether the tyres currently on your car match what the manufacturer specified, it is an easy thing to have checked. The team at Global Tyres Geelong in North Geelong is happy to read the placard against your sidewalls and talk through the right fitment before you next need new rubber.