Tyre guide

Why are tyre widths measured in millimetres and the wheels in inches?You can thank the radial revolution for this. Prior to the development of radial tyre construction by Michelin in the Forties, the major manufacturers were American and British, and they worked in imperial measurements. Michelin chose the metric system for the new technology partly because it had used it previously, but also to distinguish it from the existing cross-ply construction. As the firm’s hi-tech tyres went on existing rims, the imperial sizes remained for this reason. But wheels which use new technology such as the PAX run-flat system are measured in millimetres.

When replacing only two tyres, the advice used to be to put the new ones on the front. Now we are told the replacement rubber should go on the back. Why?Both theories only really apply when driving in the rain. The first was thought to reduce the risk of aquaplaning – when the tyre rides up on to the water rather than cutting through it – thus giving drivers better steering control. Nowadays, it is believed that the risk is greater if you lose control of the rear of the car. Most motorists can cope better with a front-wheel skid than when the back slides out, so the grippiest rubber should go on the rear.Is a repaired punctured tyre as good as a new one?The short answer is no. Any damage to the tyre’s construction compromises how well it will work, particularly at the limit of its performance. That’s not to say it is dangerous; it’s simply not as effective. There are strict rules on repairing tyres. The British Standard dictates precisely where rubber can be patched up, and limits the size of the damage that can actually be fixed. Providing these rules are adhered to, the tyre should be fine.

I’m planning to store my tyres (on rims) for around 18 months. Will they perform the same when I put them back on?As long as you keep them in a cool, dry place and out of direct sunlight, they should be fine – although performance does drop off with age. Tyres contain anti-ageing agents, but these only work when the rubber is in use; storage accelerates the decline. This is a major problem for spares, as they can often lay idle for years.

What would the difference in ride, comfort, noise and grip be between 17 and 18-inch tyres?The critical factor here is the sidewall height rather than the size of the rim – although generally, the bigger the tyre, the lower the profile. The shorter the sidewall, the less flexible it is, so steering will be sharper and there will be less roll. On the downside, theride will be harsher and it’s likely to be more noisy, too; rubber is pretty good at absorbing sound, but low-profile tyres have less of it. If you are contemplating an upgrade from 17 to 18 inches and find the ride too harsh, don’t be tempted to drop the pressures to soften it up, as this could lead to sidewall failure.

We read a lot about low-rolling resistance tyres these days, as they are more fuel efficient. Do they have less grip than regular rubber in both the wet and dry?When tyres like Michelin’s Energy and Continental’s EcoContact first appeared on the market, one of the ways lower rolling resistance was achieved was by hardening the compound – which led to reduced grip on wet roads. Development of silica in the compound has not only improved rolling resistance, but also grip in the rain and at low temperatures. Nowadays, just about all car tyres use silica, so there are no specific low-rolling resistance models as all tyre manufacturers work to keep this to a minimum.

The driver’s side tyres on my car don’t seem to be able to hold their pressures as well as the passenger side’s. Any idea why?Unfortunately, there is no technical explanation to explain this. You need to look elsewhere on the car. Check that there are no problems with the valves. Also get the wheels looked at, as they may have become porous over time or corroded around the bead, which would prevent the tyre sitting and sealing properly.

My car needs V-rated tyres, but I’d like to put W-rated ones on the back. Is this okay?Ideally, the tyres should be the same all-round, in order to maintain the handling and braking balance. But there is no problem in opting for a higher-rated tyre – just don’t be tempted to go for one that is lower than required. There will be very little difference between the same make of rubber in the next rating up or down, although this could lead to issues close to the limit. With some tyres capable of 150mph-plus, and given that the maximum speed limit across the UK is 70mph, speed ratings are more of a performance index than anything. For each tyre, this should be as closely matched as possible.

I’ve often seen ‘XL’ stamped on tyres. What does this mean, and is there a performance advantage?XL stands for Extra Load – or is sometimes referred to as Reinforced. Essentially, tyres bearing this stamp are designed for heavy cars rather than ones that carry big loads. Not only are they constructed to cope with weight, but also the higher tyre pressure these vehicles need to run at. This marking is usually found on lower-profile tyres used on large cars. It highlights the fact they are built to a higher weight rating, as there is nearly always a standard version in the same size in the maker’s range.

The price of tyres for my Audi ranges from £108 online to £200 at the main dealer. Why is there such a difference?There are a number of reasons why tyres prices vary. The main one is where you buy. A big chain that buys large volumes can demand lower prices and buy direct from the manufacturer. Independent outlets, on the other hand, often have to deal with a chain of distributors and wholesalers – all of which take a cut – which increases the final price. As for the price quoted on the Internet, an online company is likely to have much lower overheads than a regular retailer, which is why it can reduce the price considerably.I got a puncture on my Hyundai Santa Fe, and replaced the tyre with the same size as the rest. But the weight rating was 102 instead of 103. Now, the car pulls to the left when this tyre is fitted on the front. Could the different ratings be to blame?Providing that both ratings are right for the car, it is unlikely this is the cause. It may just be a rogue tyre. All rubber will pull to one side, but usually it’s unnoticeable. The effect may be greater on this particular tyre. A slight difference in tread depth between the new and old rubber can cause problems – if significant, this may be causing the transmission to wind up and affect the steering.Why can it take weeks to buy a replacement run-flat tyre? This is probably more to do with a rare size than the tyre being a run-flat. In fact, run-flats should be simple to get hold of as there are only a handful of sizes, and it is easier for makers to predict use. Problems often arise when drivers go for a non-standard wheel option, such as an 18-inch when the model usually comes with 17s. Here, the volumes available may be low – stocks could be limited if the tyres are shifted much faster than the manufacturer anticipated.