Tire Terminology Explained
Searching for replacement tires or buying tires online can be a minefield of confusing terminology and technical terms. It can all be a bit overwhelming and you may be too embarrassed to ask for an explanation in person at a garage. Fear not! These are the most common technical terms, explained in a language anyone can understand:
aspect ratio – This means the difference between the width of the tire and the height of the sidewall (which is, unsurprisingly, the sidewall of the tire). The higher the ratio, the “taller” the tire will be. A taller aspect ratio makes for a smoother ride and provides better grip in the snow. A lower aspect ratio usually comes with “low profile” tires that can be used on performance cars – they have great handling and grip, but can also leave you with a rougher ride.
contact patch – The contact patch is the small portion of your tire’s tread that actually makes contact with the road at any given time. Sports tires are much wider, so they have a larger contact patch, giving them extra grip when cornering and quicker acceleration.
Tread wear indicators – Also known as ‘wear bars’, these are the little bands or ‘bridges’ that go between the tread of your tire. When the tread wears down, they start to become visible, giving you a visual indication that your tires need to be replaced.
speed rating – If you look at the side of your tire, you will see a bunch of symbols and digits. The speed index is the letter that usually goes at the end of this data and refers to the maximum speed that your tire is capable of. Most of today’s model family cars have a speed rating of S or T (allowing top speeds between 112 and 118 mph). High performance cars may have higher ratings such as V or ZR (which allow speeds up to 149 mph or higher).
Maximum Cold Inflation Load Limit – A bite, but all this means the maximum load that the tires are capable of supporting and the maximum air pressure required to support it. This information (your recommended cold inflation load limit) should be in your vehicle manual. Overloading your vehicle and/or under or over inflating your tires can be dangerous and affect the handling of your car. The ‘cold’ aspect of the terminology refers to the fact that you should always check the pressure in your tires when they are cold. Checking them too soon after driving when they are hot means you will get a false reading as the heat will increase the pressure inside the tire.
load index – The load index is the weight capacity of the tire. You should try to use tires that have a load rating similar to the tires that came with the vehicle, or the rating recommended in your vehicle manual. The higher the number, the greater the load it can carry.
Radial and Bias Tires The difference between these two types of rims comes down to the way the cords or ‘plies’ inside the rim are ugly. Radial tires are found on most modern cars as they are more fuel efficient and provide good handling and heat dissipation. However, you may find bias ply tires on older or older vehicles, or some RVs. You should never mix radial and bias-ply tires on the same vehicle, as this will dangerously affect handling.
Temporary Use Tires – Often referred to as “space saver” tires, these are smaller-than-usual replacement tires designed to fit easily under your chassis or trunk in the event of a puncture. They are also easier to handle than full size tires. However, most space savers are not intended for use above 55mph and are only supposed to be used to get you from the roadside to a garage so you can fit a suitable replacement tire.
Wear, traction and temperature ratings – These are ratings to provide information about the average life of your tire and its ability to stop on wet roads and dissipate heat. The treadwear index, a three-digit number, gives you an idea of how long your tire should last, although this also depends on the type of driving style you have and the mileage you cover. Traction ratings range from AA to C, with C being a ‘marginal’ quality. Always make sure your tires have the recommended minimum traction rating for your vehicle. And finally, the temperature ratings (A through C) refer to the tire’s ability to dissipate heat when under load. A lower rating indicates a lower heat capacity and means the tire is more likely to experience heat-induced failure.
If you are ever in doubt about your tire ratings and specifications, always consult a professional for advice. Buying cheap tires online can be a great way to save money, but always buy from reputable companies and check your vehicle manual to make sure you’re buying the right type of tire for your car.