Car tyres explained: tread depth, pressure, age and reading the sidewall

Your tyres are the only part of the car that touches the road, and they're one of the most common reasons for an MOT failure. A few minutes every month will keep you legal and safer.

The legal tread depth

The minimum is 1.6mm across the central three-quarters of the tread, all the way round the tyre. Driving on an illegal tyre can lead to a fine of up to £2,500 and 3 penalty points for each tyre.

Many tyre experts recommend replacing tyres at around 3mm, because braking distances in the wet lengthen as the tread wears down. Don't wait for the legal limit.

The 20p check

Put a 20p coin into the main grooves of the tread. If the outer band of the coin is hidden by the tread, you're above the legal limit in that spot. If you can see the band, get the tyre checked. Check several places across the tyre and on each tyre, because wear can be uneven.

Tyre pressure

• Check the pressures weekly, before a journey, when the tyres are cold. Warm tyres give a misleading reading.
• Use the pressures the manufacturer gives for the load you're carrying. You'll find them in the handbook and usually on a sticker inside the driver's door frame or fuel flap.
• Under-inflated tyres wear faster, use more fuel and handle less well. Over-inflated tyres can reduce grip.

Damage and uneven wear

Look for cuts, cracks, bulges in the sidewall and objects stuck in the tread. A bulge means the tyre is damaged inside and should be replaced straight away.

If one edge of a tyre wears faster than the rest, it often points to a problem with wheel alignment, the suspension or the brakes. A garage can check this. Fixing the cause saves you money on your next set of tyres.

How old is too old?

Rubber hardens with age even when a tyre isn't used much. The RAC suggests replacing tyres every five or six years, whatever the tread looks like. The age is shown in the four-digit DOT code on the sidewall: the first two digits are the week and the last two the year. A code of 2321 means the 23rd week of 2021.

Reading the sidewall

You'll see something like 205/55 R16 91V:

• 205: the width of the tyre in millimetres.
• 55: the height of the sidewall as a percentage of the width.
• R: radial construction, which is what almost all car tyres are.
• 16: the wheel diameter in inches.
• 91: the load index, which is the maximum weight each tyre can carry.
• V: the speed rating, which is the maximum speed the tyre is designed for.

When you replace tyres, match the size, load index and speed rating in your handbook, or ask the garage to.

Before you buy

• Ask for the total price fitted, including fitting, balancing, new valves and disposal.
• Replace tyres in pairs on the same axle as a minimum. Ask the garage which axle the new ones should go on.
• Ask for the date code on the new tyres, so you know how long they have been in stock.