When to change car tyres: tread, age, damage and the coin test

Change a car tyre when any one of three things happens: the tread has worn down to the wear indicators, the tyre has grown too old, or it is damaged in a way no repair can fix. On tread, tyre makers selling in India give 1.6 mm as the legal minimum, measured across the central three-quarters of the tread all the way round, and that is the height of the small wear bars moulded into the grooves. A tyre level with those bars is finished. Grip in the wet falls away well before that, so Apollo Tyres advises replacing below 2.3 mm, and it is worth acting before the monsoon if you are close. On age, Apollo advises replacing car tyres after six years, and Michelin sets ten years from manufacture as the outer limit, with yearly checks from year five. On damage, a bulge, a deep cut that shows cords, or a puncture in the shoulder or sidewall means a new tyre. The coin test popular abroad is not a reliable gauge with Indian coins; use the wear bars or a cheap depth gauge instead.

Tread depth: the legal floor and a safer line

Apollo states the legal minimum as at least 1.6 mm in the central three-quarters of the tyre’s tread, around its whole circumference. Michelin India gives the same 1.6 mm figure. That is a floor, not a target. A new car tyre has much more: Apollo gives 7.5 mm for a new Alnac 4G. The grooves are there to pump water out from under the tyre, and as they get shallower the tyre clears less water and grips less on a wet road. That is why Apollo recommends replacing once the tread wears below 2.3 mm rather than waiting for the legal limit.

How to check tread depth

Use the wear indicators

Every car tyre sold in India has tread wear indicators: small raised bars across the bottom of the main grooves. Testing them is part of BIS certification under IS 15633, the Indian standard for passenger car tyres. Find them by looking along the bottom of a main groove; there are several spaced around the tyre. As the tread wears, the bars rise closer to the surface. When the tread is level with them, the tyre is at the limit.

Use a depth gauge

A tread depth gauge costs very little and gives you a number. Push the probe into a main groove, press the base flat on the tread blocks, and read it in millimetres. Measure in at least three grooves across the tyre and at three or four points round it. Uneven readings are information too: they show the tyre is wearing unevenly, which points to pressure or alignment.

The coin test, and why Indian coins do not work

The coin test comes from the United States, where NHTSA’s advice is to put a one-cent coin into the tread with Lincoln’s head upside down: if you can see the top of his head, the tread is below 2/32 of an inch, about 1.6 mm, and the tyre should be replaced. The test depends on one standard coin design. Apollo Tyres points out that it does not work well in India, because different coins of the same value are in circulation at the same time. There is no Indian coin you can rely on to show 1.6 mm or 2.3 mm. Use the wear bars, or spend a little on a gauge.

Age: replace on the calendar too

Rubber ages whether the car is driven or not. NHTSA notes that some vehicle and tyre makers recommend replacing tyres six to ten years old regardless of tread, and that ageing cannot be seen by looking at the tyre. The makers selling in India put it this way:

• Apollo: replace after six years.
• Michelin: from five years, have tyres checked by a professional at least once a year; replace at ten years from manufacture, including the spare.

To find a tyre’s age, read the DOT code on the sidewall. NHTSA explains that its last four digits are the week and year of manufacture: 0308 means the third week of 2008. The code may be on one side of the tyre only, so look at both. Our tyre age check does the arithmetic for you, and our guide to tyre expiry dates explains why age matters even on a tyre with plenty of tread.

Damage that ends a tyre’s life

Some damage cannot be repaired however much tread is left:

• A bulge or bubble on the sidewall. Apollo says unusual bumps or protrusions mean the tyre is unsafe and should be replaced at first sight. A bulge is a sign of damage inside the tyre’s structure, and it is not something a puncture shop can mend; our tyre bulge guide explains what causes one.
• Cracks or cuts in the tread or sidewall deep enough to show cords. Apollo lists visible cracks and cuts as reasons to replace.
• Punctures in the shoulder or sidewall. Michelin says shoulder punctures cannot be repaired and sidewall punctures generally cannot.
• A tyre driven flat. Michelin warns that running on a flat or very soft tyre can damage it inside, and a professional should inspect it before any decision to repair.

If a tyre loses air repeatedly, or you feel a new vibration after a hit, have it inspected off the wheel. Our puncture repair guide explains what a shop can and cannot fix.

Uneven wear is a warning

A tyre worn on one edge, in the centre only, or in patches is telling you something about the car, not just the tyre. Fix the cause, whether pressure, alignment or balance, when you replace the tyre, or the new one will go the same way. Our car tyre life guide shows how to read the patterns.

Replacing two tyres or four

Four new tyres at once keep the car’s handling even, but two is common when one axle wears faster. Replace both tyres on an axle together, never just one. Michelin recommends fitting the new or least-worn pair to the rear axle. Continental gives the same general rule, with one exception: some carmakers specify the front axle, for example on some front-wheel-drive cars, so check your handbook.

Buying the replacements

Start from the size on your car’s placard or in the handbook, or look up your model on our tyres by car pages. Match or exceed the original load index and speed rating. Before the tyre is fitted, read its DOT date: a “new” tyre that has sat in a warehouse for two or three years has already used up part of its life.