Looking for replacement tyres or buying tyres online can be a minefield of confusion terminology and technical terms. It can all be a bit overwhelming, and you may feel too embarrassed to ask for an explanation in person at a garage. Fear not! Here are the most common technical terms, explained in a language anyone can understand:
Aspect Ratio - This means the difference between the tyre's width and height of the sidewall (which is, as you might expect, the side of the tyre). The higher the ratio, the 'taller' the tyre is. A higher aspect ratio makes for a smoother ride and provides better grip on snow. A lower aspect ratio usually comes with 'low profile' tyres which might be used on high-performance cars - they have excellent handling and grip, but may also leave you with a harsher ride.
Contact Patch - The contact patch is the small portion of your tyre's tread that actually makes contact with the road at any one moment. Sport tyres are much wider, so have a bigger contact patch, which gives them extra grip when cornering and faster acceleration.
Treadwear indicators - Also known as 'wear bars', these are the little bands or 'bridges' that go between the tread on your tyre. When your tread wears down, they begin to become visible, giving you a visual indication that your tyres need to be replaced.
Speed Rating - If you look on the side of your tyre, you'll see a whole load of symbols and digits. The speed rating is the letter that usually comes towards the end of this data and refers to the maximum speed your tyre is capable of. Most current models of family car have a speed rating of S or T (allowing for top speeds of between 112 and 118 mph). High performance cars may have higher ratings, such as V or ZR (allowing for speeds up to and above 149 mph).
Maximum Cold Inflation Load Limit - Bit of a mouthful, but all this means is the maximum load that the tyres are capable of carrying, and the maximum air pressure required to support it. This information (your recommended cold inflation load limit) should be in your vehicle's handbook. Overloading your vehicle and/or under/over-inflating tyres can be dangerous and affect your car's handling. The 'cold' aspect of the terminology refers to the fact that you should always check your tyre pressure when your tyres are cold. Checking them too soon after driving, when they are warm, means that you will get a false reading as heat will increase the pressure inside the tyre.
Load Index - The load index is the weight capacity of the tyre. You should try to use tyres that have a similar load rating as the tyres that came with the vehicle, or the index recommended in your vehicle's handbook. The higher the number, the larger the load it can carry.
Radial and Bias-ply Tyres The difference between these two types of tyre comes down to the way the cords, or 'plies' inside the tyre are laid. Radial tyres are found on most modern cars as they are more fuel efficient and provide good handling and heat-dissipation. You might find bias-ply tyres on antique/older vehicles, or on some RVs, however. You must never mix radial and bias-ply tyres on the same vehicle as this will dangerously affect your handling.
Temporary Use Tyres - Often known as 'space-saver' tyres, these are smaller than usual spare tyres meant to fit easily underneath your chassis or boot in case of a flat tyre. They're also easier to handle than full-size tyres. However, most space-savers aren't meant to be used at over 55 mph and are only supposed to be used to get you from the roadside to a garage so you can fit a proper replacement tyre.
Treadwear, Traction and Temperature Ratings - These are ratings to provide information on the average lifespan of your tyre, and its ability to stop on wet roads and dissipate heat. The treadwear rating - a three digit number - gives you an idea of how long your tyre should last, although this also depends on the type of driving style you have and mileage you cover. Traction ratings range from AA to C, with C being 'marginal' quality. Always ensure your tyres are the minimum traction rating recommended for your vehicle. And finally, temperature ratings (from A to C) refer to the tyre's ability to dissipate heat when under load. A lower rating indicate a lower capacity for heat, and mean the Suspension is more likely to suffer heat-induced failure.
If you're ever in doubt about the ratings and specifications of your tyres, always consult a professional for advice. Buying cheap tyres online can be a great way to save money, but always buy from reputed companies and refer to your vehicle's handbook to ensure you are purchasing the right type of tyre for your car.
