The first number in a tyre size is the width, also called the section width. It is simply how wide the tyre is, measured in millimetres across its widest point. In 205/55 R16, the 205 means the tyre is 205mm wide.
How width is measured
Section width is measured from sidewall to sidewall at the widest part of an inflated, unloaded tyre, not just across the tread blocks. It is always given in millimetres, in steps of 10 or 5, common car widths run from around 155mm on small city cars to 255mm and beyond on performance and larger vehicles.
What width does for a car
A wider tyre puts more rubber in contact with the road, which can improve dry grip, cornering stability and braking. That is why sportier cars and higher trims tend to run wider tyres. The trade-offs are real, though: wider tyres usually generate more road noise, add a little rolling resistance and fuel use, and can be more prone to aquaplaning because there is more tyre to push standing water out from under.
A narrower tyre does the opposite. It is generally quieter and more efficient, cuts through water more easily, and often suits winter conditions, at the cost of some outright dry grip.
Width is not a free choice
Width has to work with the rest of the car. A wider tyre needs to clear the wheel arch, suspension components and steering at full lock, and it has to suit the width of the wheel it is mounted on. Fitting a tyre that is too wide for its wheel, or too wide for the arch, leads to rubbing, uneven wear or fouling.
For this reason the car maker specifies a width to match the wheels, bodywork and intended balance of the car. That recommended width appears on the placard inside the driver's door and in the handbook.
Changing width safely
Moving to a different width is possible as part of a considered size change, but it cannot be done in isolation. A wider tyre usually needs a lower profile to keep the overall diameter close to standard, so the speedometer stays accurate and the rolling circumference is maintained. The replacement must also keep the correct load index and speed rating, and physically clear the car. Working through a proper size comparison, rather than simply fitting something wider, is what keeps the change safe and legal. Once the right width is settled, the set can be bought and booked in for fitting at a well-stocked tyre retailer such as Tyres.co.uk.
From the workshop: wider looks the part, but we see the downsides too, more tramlining on poor surfaces, more road roar, and arches that catch over bumps when someone has gone a size too far. If a car came on a sensible width, there is usually a good reason for it.
Common questions
What is tyre width?+
Tyre width, or section width, is the distance across the widest part of the tyre in millimetres. It is the first number in a tyre size, so a 205/55 R16 tyre is 205mm wide.
Is a wider tyre better?+
Not always. A wider tyre can improve dry grip and braking, but it tends to add road noise, slightly raise fuel use and can be worse in standing water. The right width is the one the car was designed around.
Can I fit a wider tyre than standard?+
Sometimes, within limits, if the wider tyre clears the wheel arches and suspension and keeps a similar overall diameter and the correct load and speed ratings. The car maker's recommended width on the door placard is the safe baseline.
Does tyre width affect fuel economy?+
A little. A wider tyre generally has slightly higher rolling resistance and can use marginally more fuel than a narrower one, while a narrower tyre cuts through water more easily. The effect is small compared with pressure and driving style.