Tyres are a great deal more than the black round things most of us consider them to be. They are rated according to the load they can carry and they also have a speed rating or index. It’s vital to get tyres with the correct speed and load rating.
What are tyre speed ratings?
This is the maximum speed your tyre is legally approved for. The tyre’s speed rating usually matches or slightly exceeds the manufacturer’s stated official top speed for your car. The difference between speed rating categories can be up to 6mph (10km/h). Speed ratings are calculated in kilometres per hour, hence the unusual increments when they’re translated to miles per hour.
Why is your tyre speed index important?
You can have a tyre with a higher speed rating than is specified for your car. However, there’s not much point doing this. You’ll simply be paying for tyre technology that your car’s performance won’t allow you to access.
Having a speed rating that’s too low is a bad idea too. It will mean that the tyre isn’t designed to cope with your car’s top speed. There are tyre speed rating insurance implications too. It might result in tyre failure and if you crash because the incorrect tyre fails, your insurer may not pay out.
Factors to consider when buying tyres
We don’t live in a perfect world and frequently you have to replace a single tyre. It’s important to know whether you can mix speed ratings on a car’s four tyres. The answer is a simple no. You should maintain the same speed rating across all your car’s tyres.
The new tyre should also ideally be the same kind of tyre as the other one on that axle.
Check your car manufacturer recommendations
Your car’s manufacturer will recommend the correct speed rating for your car. You should be able to find it in the car’s user manual. Make sure you’re looking at the correct power output. More powerful cars will probably require higher speed ratings.
The easiest way is to look in your car’s user manual. It should tell you what the tyre speed rating is for your car. It’s important that you take the speed rating for the exact model and power output of your car.
Decoding tyre speed index
Speed ratings are shown in the form of a single character. These are as follows in this tyre speed rating table:
Z: 150+ mph
It’s usually the case that the wider the tyre is, the higher the speed rating will be. These are tyres that are designed for very high-performance cars. Read more about tyre speed indexes on this government web page.
Your tyre will have a sequence of letters and numbers on the side. These will look something like 205/55 R17 90V. The last letter is its speed index. Each letter relates to a different top speed for your vehicle.
Matching the speed rating to your needs
It’s not a case of thinking you never drive above 70mph so you don’t need a tyre with a speed rating that’s higher than the lowest (Q at 99mph). Equally it’s important to get the correct speed rating to your car because it’ll be a tyre that can keep up with your car’s performance.
That isn’t just top speed, it’s also traction, cornering, comfort and wear. The tyre (and its speed rating) that your car maker recommends will be capable of matching the performance of your car.
Buying the right tyres
It’s vital to buy the right tyres for your car and then keep them well maintained. Rather than getting tyres with a lower speed rating because they’re cheaper, it’s better to buy tyres with the correct speed rating but from a budget brand.
I’ve been writing about cars and motoring for more than 25 years. My career started on a long-departed classic car weekly magazine called AutoClassic. I’ve since pitched up at Autosport, Auto Express, the News of the World, Sunday Times and most recently the Daily Telegraph. When I’m not writing about cars and motoring, I’m probably doing some kind of sport or working in my garden.