How do you choose tyre width for the season?

Heated arguments often break out on car forums over which tyre width is better for which season: some say wide tyres are always better, others praise narrow ones, and still others choose a different width for winter and summer. Who is right, and what does tread width actually affect?

First of all, I would recommend reading the article on the pros and cons of wide and narrow tyres for summer - it sets out what tyre width actually means, what range you can choose it from, and the reasoning behind buying wide or narrow tyres. But that is for summer. What about winter? Do you need to change tyre width from one season to another? Let us try to work it out.

So, in reality, all else being equal, tyre width only affects one parameter: the size of the tyre’s contact patch with the road surface. What does that mean? At the very least, it means that on a narrower tyre, more of the car’s weight falls on every square centimetre of the contact patch. In other words, the wheel is “pressed” against the road with more force per unit of area.

Does that mean the car’s stability on the road will improve, and braking distance, correspondingly, will get shorter? This is exactly the point where we need to talk about the quality of the road surface.

If the road is dry, the difference between a narrow and a wide tyre will most likely be minimal, and if anything it will favour the wide tyre. After all, its contact patch is larger, and the force acting per unit of contact-patch area is not as critical, mainly because of the very high friction coefficients between rubber and tarmac.

It is a different matter if the road is wet, or covered in ice or snow. In this case the forces acting as a wedge to push the tyre out (water, snow, ice - see the article on the aquaplaning effect for more) act on a smaller area and meet greater resistance in the form of the mass bearing down per unit of area. So, if there is something else between the tarmac and the tyre, a narrow tyre will work noticeably better.

Off-road terrain. Off-road terrain usually means marshy or snow-covered ground that you nonetheless need to get across somehow. In general we are talking both about unstable terrain and about loose (viscous) ground. Getting across difficult terrain is directly tied to the wheelbase and ground clearance and depends little on the tyres themselves (unless we are talking about changing ground clearance by fitting larger-diameter wheels), whereas successfully getting across loose, viscous ground (mud, loose snow and so on) depends mainly on the tyres - or, more precisely, on their width and tread pattern.

Military drivers are taught that if a car gets stuck in mud you should let the tyres down. Why, do you think? That’s right: a deflated tyre “spreads out” under the car, and its contact area with the “road” grows several times over. As a result, the weight per unit of area falls sharply, and the chances of getting out of the mud go up. Why does this happen? For the reasons described above: in loose ground, a wedge forms in front of each wheel that works to push it out, and if the contact patch is bigger while the load per unit of area is lower, the force of that push increases.

To sum up: does it make sense to change tyre width by season?

Given how well our road-clearing services do their job, in winter courtyards, car parks and even city streets often turn into off-road terrain. In these conditions a wide tyre will give you a slight advantage. But you can get that same advantage just by letting a narrow tyre down a little, if you run into trouble getting out of a yard or a garage.

A narrower tyre, on the other hand, delivers consistently higher stability and shorter braking distances on a slippery, hard surface.

Which matters more to you is for you to decide, but it is precisely the behaviour of tyres of different widths in typical summer and winter conditions described above that underlies the general recommendation: it is better to choose winter tyres a little narrower than summer ones. That said, it is important to remember that your choice of tyre width should stay within the sizes the car manufacturer allows for this car model.

How does tyre width affect the profile?

The figure marked on a tyre’s sidewall for its profile shows the tyre’s height as a percentage of its width, so at the same profile figure a wider tyre will, of course, be taller and will increase the wheel’s overall diameter. In theory, then, you could get more comfort and less load on the suspension, but in reality it is not quite that simple.

The thing is, increasing the wheel’s overall diameter is, first, not always physically possible (a larger-diameter wheel can foul the bodywork), second, it distorts the speedometer reading and increases the load on the transmission, and, third, it shifts the steering axis. These are exactly the three reasons why this kind of wheel “upgrade” is, in most cases, not recommended by car manufacturers for most models.

So, to keep the wheel’s overall diameter unchanged when you change tyre width, you need to choose a lower profile for a wider tyre. For example, if the standard tyres on your car are 195/65/15, then at a width of 205 mm the correct profile is no longer 65 but 60, meaning that the correct wider replacement for the standard tyre in this case should be the size 205/60/15.

Finding a suitable replacement for your standard tyre in a wider or narrower size without significantly changing the wheel’s overall diameter is fairly simple, using the tyre size calculator.