Tyre and Rim Association of Australia Standards Manual: How to Read the Numbers on a Tyre Sidewall

The Tyre and Rim Association of Australia Standards Manual is the Australian technical reference that defines what tyre size designations and rim dimensions mean. For a marking such as 205/55R16 91V, it sets the design section width, overall diameter, approved rim widths and load capacity behind each number. The sidewall gives a short code. The manual gives the dimensions, loads and rim details that sit behind that code.

What is the Tyre and Rim Association of Australia Standards Manual?

The Tyre and Rim Association of Australia (TRAA) is an industry standards body. Its Standards Manual records agreed engineering data for tyres, rims and valves used in Australia. Manuals of this kind typically include:

• Tyre size designations, meaning the naming format for each size.
• Design dimensions, such as section width and overall diameter. These are usually stated for a new tyre, inflated and unloaded, on a specified measuring rim.
• Rim dimensions, including the measuring rim and the range of approved rim widths for each tyre size.
• Rim contours, such as flange shapes and bead seat profiles.
• Load and inflation tables, which link load index, inflation pressure and carrying capacity.
• Valve and related component data.

Australian vehicle regulations and modification codes refer to this kind of standardised tyre and rim data. Each referenced document names the specific source and edition it relies on. The practical point for a reader is simple: the sidewall code is shorthand, and the manual turns it into dimensions and loads.

How do you read the numbers on a tyre sidewall?

A standard passenger tyre size such as 205/55R16 91V combines dimensions, a ratio, a construction code and two service ratings. Each part has a different unit and purpose.

Marking	What it is	Unit or type	Plain meaning
205	Nominal section width	Millimetres	The nominal width of the tyre from sidewall to sidewall
55	Aspect ratio	Percent (a ratio, not a length)	Sidewall height is 55% of the section width
R	Construction	Code	Radial construction
16	Rim diameter	Inches	Fits a rim with a 16-inch bead seat diameter
91	Load index	Code (performance rating)	Maximum load per tyre of 615 kg under the reference conditions
V	Speed symbol	Code (performance rating)	Maximum rated speed of 240 km/h under the reference conditions

Dimensions: section width and rim diameter

Section width is given in millimetres and rim diameter in inches. One size code mixes metric and imperial units, so convert the rim diameter before calculating anything (1 inch = 25.4 mm). The section width is a nominal design value. The width you would measure on a real tyre depends on the rim it is mounted on.

Ratio: the aspect ratio

The aspect ratio is a percentage. It describes sidewall height compared with section width, so it is not a measurement in its own right. A 55-series tyre on a 205 mm section has a much taller sidewall than a 55-series tyre on a 165 mm section. To find the actual sidewall height, you have to calculate it from both numbers.

Performance ratings: load index and speed symbol

The load index and speed symbol are service ratings, not dimensions. They tell you how much load a tyre can carry and at what speed, under the specified pressure and test conditions.

Load index	Maximum load per tyre	Speed symbol	Maximum speed
91	615 kg	S	180 km/h
94	670 kg	T	190 km/h
100	800 kg	H	210 km/h
107	975 kg	V	240 km/h
109	1030 kg	W	270 km/h

A tyre reaches its full rated load only at the inflation pressure set out in the load and inflation tables. If you run it at lower pressure, it can safely carry less.

Other size formats: XL, C, LT and flotation sizes

• XL or Reinforced: an extra-load passenger tyre. It has a higher load index than the standard version of the same size, and needs higher inflation pressure to reach that load.
• C (for example 215/70R15C 109/107R): a commercial or light-truck metric tyre. The two load indices give the capacity in single and dual fitment.
• LT: a light-truck designation. It uses its own load and inflation tables.
• Flotation sizes (for example 31x10.50R15LT): overall diameter about 31 inches, section width about 10.50 inches, rim diameter 15 inches. In this format, every dimension is given in inches.

What do rim dimensions such as 6.5J x 16 mean?

A rim marking such as 6.5J x 16 describes a wheel with a 6.5-inch rim width, a J-type flange contour and a 16-inch rim diameter. Rim width is measured between the inside faces of the flanges, not across the outside of the wheel.

For each tyre size, the standards manual lists a measuring rim, which is the rim width used to define the design dimensions. It also lists a range of approved rim widths. A tyre mounted on a rim outside that range may not seat or perform as designed. Within the range, a wider rim generally makes the measured section width slightly larger, and a narrower rim makes it slightly smaller.

How do you calculate sidewall height, overall diameter and circumference?

You can work out a tyre's calculated geometry from the size code in four steps. The worked example uses 205/55R16.

1. Sidewall height = section width × aspect ratio ÷ 100. Example: 205 × 0.55 = 112.75 mm.
2. Rim diameter in millimetres = rim diameter in inches × 25.4. Example: 16 × 25.4 = 406.4 mm.
3. Overall diameter = rim diameter + (2 × sidewall height). Example: 406.4 + 225.5 = 631.9 mm.
4. Circumference = overall diameter × π. Example: 631.9 × 3.1416 ≈ 1985 mm, or about 504 revolutions per kilometre.

The result is a geometric estimate from the nominal code. It is not a guaranteed measurement of any particular tyre.

How do width, aspect ratio and rim diameter change overall size?

Overall diameter depends on all three numbers together. A larger rim diameter, a wider section or a higher aspect ratio each tends to increase sidewall height or overall diameter, but a change in one can be offset by a change in another. That is how "plus sizing" works: you fit a larger rim with a lower-profile tyre so the overall diameter stays close to the original.

Size	Sidewall height	Overall diameter	Circumference	Diameter difference vs 205/55R16
205/55R16	112.75 mm	631.9 mm	≈1985 mm	Reference
225/45R17	101.25 mm	634.3 mm	≈1993 mm	+2.4 mm (about +0.4%)
215/60R16	129.0 mm	664.4 mm	≈2087 mm	+32.5 mm (about +5.1%)

What the differences mean in practice

• 225/45R17 has a shorter sidewall than the reference size, but the larger rim almost exactly makes up for it. The calculated overall diameter is only about 0.4% larger. A speedometer calibrated for the original size would read roughly 100 km/h when the true speed is about 100.4 km/h.
• 215/60R16 keeps the same rim but has a taller sidewall. The calculated diameter grows by about 5.1%, which raises the axle by about half the diameter difference (roughly 16 mm). At an indicated 100 km/h, the true speed would be about 105 km/h. The larger diameter also reduces clearance to bodywork and suspension.

Why calculated geometry differs from actual tyre dimensions

A calculated diameter is derived from nominal numbers, so it will not exactly match the measured size of a real tyre. Standards manuals publish design dimensions and allow tolerances, and actual tyres differ for several reasons:

• Brand and model: two tyres with the same size designation can measure slightly differently within the permitted tolerances.
• Rim width: the design dimensions assume the measuring rim. Other approved widths change the section width and sidewall shape.
• Load and pressure: under vehicle weight, the loaded radius is smaller than half the unloaded diameter, and the effective rolling circumference is shorter than π × diameter.
• Tread wear: a worn tyre has a smaller diameter than a new one.
• Growth in service: tyres can grow slightly after inflation and use.

Use calculations to compare sizes against each other. Use the manufacturer's published dimensions, or a measurement, when exact clearance matters.

Does a matching calculated diameter mean a tyre will fit the vehicle?

No. A close overall diameter is only one part of fitment compatibility. Before changing size, check every one of the following:

1. Vehicle tyre placard: confirm the sizes, minimum load index, minimum speed rating and pressures the vehicle manufacturer specifies.
2. Load index: the replacement tyre must carry at least the required load at the pressure you will actually run.
3. Speed symbol: it must meet the applicable requirement for the vehicle.
4. Rim width: the wheel width must be within the approved range for the tyre size.
5. Physical clearance: check for contact at full steering lock and full suspension compression. Wheel offset matters as much as tyre size here.
6. Vehicle systems: diameter changes affect speedometer and odometer readings, and can affect electronic stability and ABS systems.
7. Regulations: Australian state and territory modification rules limit changes to tyre and wheel size. Check the current rules for your jurisdiction before fitting non-standard sizes.

Where can you get the Tyre and Rim Association of Australia Standards Manual PDF?

The Standards Manual is a copyrighted technical publication issued by the Tyre and Rim Association of Australia. The current edition should be obtained from the association itself. Free copies labelled "tyre and rim association of australia standards manual pdf" that circulate online may be old editions or unauthorised copies. Load tables, approved rim widths and listed sizes can change between editions, so check the edition date before relying on any figures.

When you use the manual, look up the exact size designation, including any XL, C or LT marking. Note the measuring rim and the approved rim widths. Then read the design dimensions and the load-inflation entry for the pressure you intend to use. Read together with the sidewall markings and the vehicle placard, these values tell you what a tyre is designed to do, and where calculated comparisons stop being enough.