Choosing the right tyres for winter A two-wheel drive car on winter tyres is better than a four-wheel-drive car on summer tyres... Having just about (and it was a close thing) managed to zig zag our way to the top of the Ice Hill in a four-wheel-drive Cayenne on summer tyres, we are going to try something slightly different. Climbing down from the big SUV I land in the seat of a rear-wheel drive, high-performance Porsche Boxster and return to the bottom of the Ice Hill. In theory the Boxster shouldn't stand a chance here, but as I slot the gearlever into drive and lean on the throttle it begins to move. Be too ambitious with the power and the rear wheels will still spin, but with a bit of restraint it ascends the hill with ease, its steering crisp and controlled. The difference, of course, is that it's wearing winter tyres. ...but a four-wheel-drive car with winter tyres is better again With that experience under our belt it's no surprise to find that swapping to our third and final car, a four-wheel-drive Cayenne on winter tyres, we make it to the top of the hill with ease. In fact, such is the grip on offer that you have to be quite aggressive with the controls for anything dramatic to happen. It's a similar story when we put the cars through a slalom, this time travelling down the Ice Hill. On summer tyres the Cayenne doesn't even make the first turn, understeering hopelessly before threatening to go into a huge spin. Try the same in the Boxster on cold-weather tyres and you still need to work the steering, applying half a turn of opposite lock as you guide the car between the obstacles, but the difference is that as soon as you straighten the steering wheel the tyres regain grip. Again though, it's the Cayenne on winter tyres that puts in the strongest performance, weaving through the slalom without a problem. And it's not just that you can see and feel the grip, but you can hear it too, the tyres thrumming merrily as they bite into the surface. Four-wheel-drive might help to you get moving, but it's of no use when you're trying to stop Our driver in the Cayenne on summer tyres might have had a torrid time getting to the top of the hill, but now there he can continue on his way. Before he knows it he is heading back downhill again at a steady 24mph when, in the distance, he sees a lorry has jack-knifed and is blocking the road. He brakes as hard as he can, but while he can feel the ABS system pulsing through the pedal the car sails forward, its speed barely diminishing as the tyres glide across the road surface. On that same seven per cent Ice Hill it takes 55 metres to come to a complete stop. From the driver's seat it is, quite frankly, terrifying. Behind, the Cayenne with winter tyres is travelling at exactly the same speed. Spotting the lorry at the same point it too performs an emergency stop. Again the ABS pulses, but this time the nose dives as the tyres grip, and the car comes to a halt in just 30 metres. In fact, so effective are the tyres that it takes the Cayenne just five metres more to stop than the much lighter Boxster. What makes winter tyres so effective? First, the tread pattern. Winter tyres feature more "sipes" (or grooves) within the tread blocks (Michelin's winter tyres feature about 1,500 sipes, compared with 200 for a normal tyre) to provide traction and stability. In snow these sipes actually fill with snow, which then grips to the snow on the road (imagine rolling a snowball) to provide traction. There will also be a higher groove to rubber ratio to help clear water and reduce aquaplaning. Winter tyres also have a different compound, with a high-silica content meaning that they stay softer, more pliable and thus offer more grip at low temperatures than summer tyres. When searching for new rubber, SUV drivers might also encounter Mud and Snow tyres, which are defined by TyreSafe as those "whose tread and structure are designed to give better handling than normal tyres in slush and fresh or melting snow." However, there are no rules stating that such tyres should also feature a winter-specific compound, which is so essential to performance in cold conditions. To ensure that the tyres are winter-specific, look for a snowflake or snow-topped mountain symbol on the sidewall. When should I fit winter tyres and how much will they cost? Due to their compound, winter tyres come into their own as soon as the temperature drops below seven degrees C. That is to say, from late October until March you'll benefit from superior acceleration, braking and handling - and thus safety - if your car has winter tyres fitted. That applies on dry, clear roads, and their advantage over summer tyres only increases in rain, snow and ice. As for cost, it depends on the car you drive and the tyre you choose to buy. For the Cayenne we've been testing, an approved wheel and winter tyre package from Porsche costs from £1,600, and for the Boxster from £1,800. Audi sells wheel and tyre packages from £799 (or tyres only from a very reasonable £325), and at present is throwing in a year's roadside assistance if you buy wheels and tyres. The other option is to source your own winter rubber, for which you'll be paying about £350 plus £40-60 for fitting, depending on which tyres you choose and where you take them to be fitted. For the latter, it's well worth phoning around for a few quotes. Remember too that the performance of a summer tyre will vary enormously depending on how much you spend, and the same applies to winter rubber. Other considerations Last winter the Association of British Insurers published a winter tyre agreement, stating that insurers would not charge an additional premium if customers choose to fit winter tyres. As such, there shouldn't be any associated cost with making the swap, but it's worth a quick courtesy call to let your insurer know. It sounds obvious, but consider where you're going to store your existing wheels and tyres. If you don't have room at home then your main dealer might have space and be happy to store them. Typically this will cost about £10 a month, although some dealers might have special offers (Suzuki is currently charging customers £49.99 for a year). Remember that winter tyres must be fitted to all four wheels. Only fitting them to the driven wheels on a two-wheel-drive car will result in unbalanced handling and braking. If you already use winter tyres remember to check that they have sufficient tread. The AA advises at least 3mm for winter driving, and no less than 2mm.