Tyre Sizes and Pressures
Tyre Specifications - Decoding the myth
A tyre is black and round, right? Wrong!
Tyres have different tread patterns, compounds, widths, sidewall sizes, and speed ratings. This information is written on the sidewall of the tyre as a series of numbers and letters, which can be confusing and even intimidating - unless you know what they mean.
Don't despair; the numbers are actually easy to understand once you know how. You can check out our fun instructional (see right) or read on.
Let's say the large set of numbers on the sidewall of a tyre reads 215/45 R17 84V.
215 - This is the width of the tyre across its tread in millimetres. If you took a ruler and measured the tread, it would read about 21.5cm, or 215mm.
45 - This is the width of the sidewall, as a percentage of the width of the tread. This tyre's sidewall is 45 percent of the tread. If you measured it with a ruler, it would be just under 10cm. An off-road tyre or a tyre designed for comfort will usually have a higher profile number. A performance tyre, also known as a 'low profile' tyre, will have a smaller profile.
R17 - The R stands for radial; this is a radial road tyre. The 17 is the diameter of the wheel or rim in inches, therefore this tyre is designed for a 17-inch rim. Want to check what size your rim is? The size is typically stamped inside the rim on one of the spokes, or you can measure across the spokes from one inside edge to the other.
91W - this is the load index and speed rating of the tyre. Essentially, the load rating, represented by the numerical value, is related to the maximum weight that tyre can carry. The tyres load index must meet or exceed the value displayed on the vehicle's tyre placard (see below). The speed category symbol, represented by the alphabetical value, indicates the maximum speed at which the tyre can carry a load corresponding to its load index.
Finding the right load and speed rating for your vehicle is a bit trickier, as you need to cross-reference these values with a chart to find the right tyre. The best thing to do is ask the experts to check your tyre is correct for your vehicle and what you plan to do with it.
The Tyre Placard
It's important to have the right tyres fitted to your vehicle for safety reasons as well as from both a legal and insurance perspective.
This is the tyre placard from a Subaru Impreza.
It shows 3 different the 3 different tyre sizes available across the range. The 2009 Subaru Impreza WRX which has this placard is fitted with tyre option B with a 225/45R17 tyre size and a 91 load rating with a W speed rating.
The placard says that if the car is loaded with 3 people and a small amount of baggage or cargo the front tyres should be inflated to 230 kPa (That's about 33 psi). The rear tyres should be inflaed to 220 kPa (That's about 32 psi).
If the Subaru Impreza was loaded with 5 adults and full of luggage it would need more tyre pressure to carry the additional weight. So the recommended tyre pressure for the rear would increase from 220 kPa to 230 kPa.
If the Subaru was towing a trailer the rear tyre pressure would be increased to 250 kPa (That's about 36 psi).
The Subaru Impreza has its tyre placard in the driver's door jam.
A Holden Barina has its tyre placard on the glovebox door.
Since January 1973, new vehicles have been required by law to have a tyre placard affixed to the car, usually on the driver's door pillar or inside the fuel filler cap.
This placard will tell you the factory fitted tyre sizes and recommended tyre inflation pressures for the vehicle. You should locate the placard on your vehicle and become familiar with this important information.
Many components go in to a modern tyre to give it the performance, handling and safety that we've come to expect.
For instance, the bead wires have the primary task of holding the tyre firmly on the rim while the body plies provide the strength to constrain the forces created by the internal air pressure. The steel belts work in conjunction with the bead areas to maintain the tread in full contact with the road surface allowing the tread area to perform its main task of providing grip on both wet and dry surfaces of all types.
The tyre is a package designed to give you tens of thousands of kilometres of safe, comfortable driving.
Think of tyres as your car's shoes, and the tread as the sole.Some shoes have hard, smooth soles and are treacherously slippery. Others have moulded patterns and rubber that grips to give you good traction.
The grooves in the tread are specially designed to remove water from under the tyre in wet conditions. As the tread wears down, the tyre's ability to remove water gradually reduces. Consequently, a 'bald' tyre is very dangerous in the wet.
The minimum legal tread depth on a tyre is 1.5mm - about the height of the head on a match. On the side of your tyres you can find little arrows pointing to tread wear indicator bars. When the tread wears down to this level, the tyres need replacing.
Have you ever noticed your tyres (particularly the front) wearing on the edge but not across the tyre? Tyres showing this, or even feathered edges, may indicate the wheel alignment needs adjusting. Most tyre services recommend a wheel alignment every 10,000km.
The front (and sometimes the rear) wheels on a vehicle need to be adjustable to allow for variation in vehicle control characteristics. This can only be done by a technician using a wheel alignment machine. There are various types of wheel alignments; the alignment you require depends on the make of vehicle you are driving.
Correct wheel alignment will:
- Extend the life of tyres by reducing premature wear
- Allow tyres to wear more evenly
- Improve the handling of the vehicle
- Optimise the steering response of your vehicle
Regular tyre rotation will help to lengthen the life of your tyres as well as reduce any uneven wear. Tyre rotation is important for all vehicles, but especially for front-wheel-drive models where the front tyres have to do a lot of work. Remember to consider the spare in rotation. The spare should go to the right rear position; then the tyre in the diagrams shown as moving to the right rear should become the spare. Tyre pressures need to be checked after rotation.
The recommended rotation is illustrated in the diagrams below for both front and rear-wheel-drive vehicles. It is much easier for your tyre specialist to perform this for you than for you to try it with limited equipment. It's also a good opportunity for them to keep an eye out for any signs of mechanical problems before they get worse.