During the ride, the surface of the tread, depending on the compound used, wears out at a different pace to such an extent that the tire no longer complies with the law standards. However, those standards were put in place for a good reason – a worn tire has poor grip, a longer braking distance and is more prone to aquaplaning, so regular tread inspection is an absolutely indispensable habit for every driver. So let’s summarize the ways in which we can determine the depth of tyre tread at home.
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What needs to be done before the measurement itself?
For the most accurate measurement, it is first necessary to stop the car on a leveled surface. It is advisable to turn the front wheels with the steering wheel to the largest possible angle so that we can get to them better. Once the car is ready, we will try to clean the grooves of the tread from dirt in the places where we want to measure the depth, otherwise the result will be logically distorted. Before measuring, it is also advisable to recall the required standards, for summer tires a minimum tread depth of 1.6 mm is required and for winter tires a depth of 4 mm is required.
Tread Wear indicator (TWI)
The easiest way to determine a worn out tire is to look at it. The grooves in the tread of summer tires are often equipped with 1.6 mm thick rubber pads, which is the legal minimum for passenger cars. As soon as the tire wears to the specified limit (most tires have a pointer located at a depth of 1.6 mm), the pad becomes parallel to the rest of the tire, which is a sign for the driver that the tire needs to be replaced. You can find the location of the wear indicator according to the mark located near the indicator on the sidewall of the tire. Among the brands the most commonly used symbols are brand logo, abbreviation TWI (Tread Wear Indicator) or a triangle. Some winter tires are also equipped with a winter indicator at a depth of 4 mm. For those, look for a symbol of a snowflake on the side of the tread.
Some manufacturers often also use the word “TWI” on the side instead of an arrow. This picture shows a tire that should have already been replaced because it is approaching the legal limit of 1.6 mm.
Numerical wear indicator
The numerical wear indicator is an āupgradedā version of the above-mentioned TWI indicator, which shows you the tread depth without the need of measuring it with a special tool. The numbers indicate the depth of the tire and are gradually erased as the tire wears out. This is a smart system that is simple and at the same time very accurate. However, we do not encounter a numerical wear indicator for all brands, currently it is currently used by Nokian or Matador, for example. Also bear in mind that it shows the tread depth only in the middle of the tread – if you donāt have the right tire pressure or your car has alignment issues there is a great chance that your tires wear out unevenly and might need replacing even though there is enough tread left at the center of the tread.
Tread depth indicator on Nokian tires – it is obvious at first glance that there is approximately 6 mm of tread left
Depth gauge
The depth gauge is a small sliding tool that can be used to accurately analyze the tread on individual parts of the tread. Depth gauges can be purchased at any good car service or hardware store and they are pretty cheap (after all, it is just a piece of plastic with numbers on it), but you can also get a digital gauge or a professional certified tool which can be pricey.
Simple and cheap tread depth gauge works fine for most cases
Coin measurement
There is a āhackā for drivers that donāt have other options of measuring tread depth – you can use a coin. We insert the coin into the groove in the tread and according to how deep the coin gets, we determine the wear of the tires. Which coin really depends on the country you are in. In the UK, your best bet is a 20p coin – if the outer band of the 20p coin is obscured when it is inserted, then your tread is above the legal limit. In EU countries, 1 euro coin is also usable – If the gold rim of the coin is showing then your tread depth is under 3mm and ideally you should be replacing the tyre.
For winter tires, which have greater demands on the tread depth, a 2 euro coin is used, if the outer silver edge is visible, the tire is worn. However, measuring the depth of the coin design is not sufficient and cannot replace a depth gauge, these are only indicative values.
This tire has sufficient tread depth around 5 mm, but is unevenly worn out and should be replaced
One last advise
Experts agree that 1.6 mm as the minimum permissible tread depth is not enough – it is much safer to change tires if the tread depth falls below 3 mm. Below this limit, the ability to prevent aquaplaning worsens rapidly. We can have a look at data that Nokian provided:
| New summer tire – 7mm | 83.1 km/h |
| Worn-out summer tire 3-4 mm | 61.2 km/h |
| Worn-out winter tire 3 mm | 60.3 km/h |
Starting speed of aquaplaning when driving in a bend