How To Read Your Tyre Sidewall: Size, Load And Speed Ratings

Most drivers glance at their tyres every day without giving the sidewall markings a second thought. The string of numbers and letters moulded into the rubber looks like a code — and in a sense, it is. It tells you the exact dimensions of the tyre, how much weight it can safely carry, how fast it can run, and when it was made. Understanding what those markings mean is one of the more useful pieces of automotive knowledge a driver can have, particularly when it is time to replace tyres or upgrade to something better suited to how the vehicle is used.



This guide breaks down each element of the sidewall marking in plain language, explains why the ratings matter and covers the questions most drivers have when they are comparing tyres or wondering whether the ones on their vehicle are right for it.

The most prominent marking on any car tyre follows a format that looks like this: 215/55R17. Each part of that sequence tells you something specific about the tyre's dimensions.



215 — this is the tyre width in millimetres, measured from sidewall to sidewall across the tread. A wider tyre has a larger contact patch with the road, which can improve grip and handling but may also increase rolling resistance and fuel consumption.

55 — this is the aspect ratio, expressed as a percentage. It represents the height of the tyre's sidewall as a percentage of the tyre's width. A 55 aspect ratio means the sidewall height is 55% of 215mm, which works out to approximately 118mm. Lower aspect ratios (40s and below) mean shorter sidewalls, which generally improves cornering response but reduces ride comfort. Higher aspect ratios mean taller sidewalls, which absorb road imperfections more effectively.

R — this indicates the tyre's construction type. R stands for radial, which is the construction method used in virtually all modern passenger and light commercial tyres. Radial construction means the tyre's internal cords run perpendicular to the direction of travel, which provides better flexibility and heat dissipation than the older bias-ply construction method.

17 — this is the rim diameter in inches. The tyre must match the wheel it is fitted to exactly — a 17-inch tyre will only fit a 17-inch rim.

After the size marking, you will typically find a two or three digit number followed by a letter — for example, 91V. The number is the load index and the letter is the speed rating.



The load index is a numerical code that corresponds to the maximum weight the tyre can carry when properly inflated. The scale runs from around 60 (250kg per tyre) up to 120 and beyond for heavy commercial tyres. An index of 91 corresponds to a maximum load of 615kg per tyre. For a passenger vehicle, the combined load capacity of all four tyres needs to exceed the vehicle's Gross Vehicle Mass (GVM) — the maximum total weight of the vehicle including passengers, cargo and the vehicle itself. If the load index is too low for the vehicle, the tyres are operating outside their rated capacity, which increases the risk of heat buildup, tyre failure and unpredictable handling.

Load index becomes particularly important for:

• Towing — when towing a caravan or trailer, the tongue weight adds to the vehicle's rear axle load. Tyres rated for standard passenger use may be insufficient for the combined load during towing, particularly on longer trips
• 4WD and dual-cab ute owners — vehicles that regularly carry heavy loads in the tray or tackle off-road terrain benefit from tyres with a load index rated for those conditions
• Commercial van and bus operators — the difference between a lightly loaded and fully loaded commercial vehicle is significant, and the tyres need to be rated for the maximum load, not the typical empty weight

The letter that follows the load index is the speed rating. This indicates the maximum sustained speed at which the tyre is designed to operate safely when correctly inflated and carrying its rated load. Common speed ratings and their corresponding maximum speeds:

• T — 190km/h
• H — 210km/h
• V — 240km/h
• W — 270km/h
• Y — 300km/h



For most everyday drivers, the speed rating rarely comes into play as a literal speed limit — Australian roads have a maximum of 110km/h in most jurisdictions, and most driving happens well below that. But the speed rating also reflects the tyre's construction quality, heat resistance and handling characteristics at higher speeds. A higher-rated tyre is generally built to tighter tolerances and with better materials than a lower-rated equivalent.

Where speed rating matters in practice:

• Performance vehicles should always be fitted with tyres that meet or exceed the manufacturer's speed rating requirement
• If one tyre on an axle has a lower speed rating than the others, the lower rating applies to the whole vehicle — mixing speed ratings affects handling balance
• Tyres fitted below the vehicle manufacturer's minimum speed rating may affect insurance validity in the event of an accident

Somewhere on the sidewall, usually near the rim edge, you will find a four-digit number preceded by the letters DOT. The last four digits of the DOT code tell you when the tyre was manufactured — the first two digits indicate the week of manufacture and the last two indicate the year. A tyre marked 1824 was made in the 18th week of 2024. This matters because rubber compounds degrade over time even when a tyre looks visually intact and has adequate tread depth. Most tyre manufacturers recommend replacing tyres at ten years from the manufacture date regardless of condition, and some recommend inspection at six years. A tyre that has been sitting in a warehouse or on a spare wheel for years before being fitted may have less remaining service life than its tread depth suggests.

When purchasing new tyres, it is worth checking the manufacture date to confirm you are not receiving old stock. A tyre manufactured more than two years ago is generally considered to have significant shelf life used up, even if it has never been fitted.

Beyond the main size, load and speed markings, tyre sidewalls carry several other codes that are useful to understand.

M+S (Mud and Snow) — this marking indicates the tyre meets a basic winter traction standard. It does not necessarily mean the tyre is purpose-built for serious winter conditions, but it does indicate improved performance in light mud and snow compared to a standard summer tyre.

Three-Peak Mountain Snowflake (3PMSF) — this symbol (a mountain outline with a snowflake inside) indicates the tyre meets a more rigorous winter performance standard. In parts of Europe and North America, this symbol is required for winter tyre legal compliance, though it is not a legal requirement in Australia.

Reinforced (XL or RF) — this marking indicates the tyre is built with extra load capacity beyond the standard construction, typically achieved through additional reinforcing plies. XL-rated tyres are increasingly common on SUVs and vehicles with heavier payloads, and they require higher inflation pressures to achieve their rated load capacity than standard tyres of the same size.

TL / TT — TL indicates a tubeless tyre (by far the most common type in modern vehicles), while TT indicates a tube-type tyre requiring an inner tube. Fitting a tube into a tyre marked TL is not recommended.

This is one of the most common questions drivers have, and the answer requires some nuance. For tyre size, minor variations — such as fitting the next width up or a slightly different aspect ratio — are often acceptable and even done deliberately to change the ride or aesthetic of a vehicle. However, any size change affects the tyre's rolling circumference, which in turn affects speedometer accuracy, ABS calibration and stability control systems that rely on wheel speed data. Significant size changes can also affect clearances in wheel arches and suspension components. In New South Wales, tyre size modifications outside specified tolerances may affect a vehicle's compliance certificate.

For load index, fitting tyres with a lower rating than the manufacturer specifies is not acceptable from a safety standpoint and may affect insurance validity. Fitting tyres with a higher load index than specified is generally safe and sometimes done for towing applications or upgraded suspension setups. For speed rating, fitting tyres rated below the manufacturer's minimum is not recommended and may affect insurance validity. Fitting tyres with a higher speed rating is generally acceptable and is sometimes done to improve handling characteristics, though higher-rated tyres may have a stiffer ride. When in doubt, the safest approach is to consult a tyre professional who can assess your vehicle's requirements, the way you use it and whether any changes from the manufacturer's specification are appropriate.

Understanding your tyre sidewall markings is useful background knowledge, but applying that knowledge to your specific vehicle, driving conditions and use case is where professional guidance adds real value. Situations where it is worth talking to a tyre specialist include:

• Upgrading to larger or more capable tyres for 4WD, towing or off-road use
• Replacing tyres on a modified vehicle where the original specification may not apply
• Choosing between multiple tyre options that meet the size specification but differ in load and speed ratings
• Assessing whether tyres on a recently purchased vehicle are appropriate for the vehicle and its intended use
• Any uncertainty about whether a tyre size change will comply with NSW road rules or affect a vehicle's registration

We at BOEX Tyres & Performance have been helping drivers across Bowral and the Southern Highlands understand and choose the right tyres for over 40 years. Whether you need tyre maintenance in Bowral, are researching tyre maintenance in the Southern Highlands or simply want to make sense of what is moulded into the sidewall of your current tyres before making a decision, our team is happy to help. For tyres Bowral drivers and Southern Highlands locals can trust, visit our website or call us on 02 4868 2600 to book a tyre check or get advice on your next set.