It doesn’t matter if it’s summer, winter or all-season tires — the remaining tread depth is extremely important. In order to prove that point and define just what the minimum safe tread depth should be, ADAC experts conducted a test of 185/60 R14 winter tires, all in a brand-new condition but with varying tread depth.

It is common knowledge that the minimum legal tread depth for all passenger car tires in Europe is 1.6 mm.

However, the extensive ADAC research proved that the minimum legal tread depth only describes the tire’s margin of safety. For summer tires, the remaining tread depth must be no less than 3 mm, and for winter and all-season tires it must be no less than 4 mm. Smaller tread depth may become critical on wet pavement, slush or snow.

Legal regulations

The law is explicit: when your tires (whose tread depth in a new condition, as a rule, ranges from eight to nine millimeters) have worn out to the minimum allowed mark of 1.6 mm, they are no longer considered to be safe to drive on, and need to be replaced.

At the same time, however, in some of the European countries, such as Austria, winter tires with a remaining tread depth less than four millimeters are legally considered as summer tires and are even forbidden to drive on certain roads.

Make sure you check your tread depth a few times a year for your own safety and peace of mind, and remember that the 1.6 mm minimum allowed tread depth is merely a legal limit. For safe driving, tires must be replaced at 3 or 4 mm remaining tread depth.

ADAC Test Results

In order to find out how the tread depth affects the tires’ performance, the ADAC expert team conducted a test of 185/60 R14 tires in a new condition (tread depth being about 8 mm) and in a used condition (tread depth ranging from 7.5 to 4 mm).

Snow

On snow, the differences between new and used tires were the most prominent of all. The braking distance of the tires with a 4mm tread depth was 3.2 meters longer than the new tires’ at a ridiculously slow speed of 30 km/h.

There were also significant differences in traction when the tires needed it most — at the start or on inclined surfaces. A tire’s traction depends on the depth of the lamellas; the circumferential grooves «bite» into loose snow, and the edges of the lamellas ensure grip on harder surfaces.

In this instance, even the remaining tread depth of 7.5 mm, compared to the new tire, is only capable of delivering 60% of the original traction  notes the technical expert (Tiresvote.com)

Tires with a 4 mm tread depth fall short of even half the performance of the new winter tires. You can easily guess how well the tires with a minimum allowed tread depth of 1.6 mm will perform — they simply have nothing to bite into snow with.

In the course of the test, the ADAC experts tried to find out, among other things, the traction provided by summer tires on snow but the best result that they got was only a quarter of the traction provided by a new winter tire and more than 7.5 meters of extra braking distance at 30 km / h.

Wet pavement

It is important for tires to resist hydroplaning on wet pavement, i.e. the circumferential grooves and lamellas must absorb and oust as much water as possible.

Used tires, whose tread depth has been worn down to 4 mm, start floating as early as at 63 km/h, while new winter tires resist the hydroplaning effect until the needle of the speedometer reaches the mark of 87 km/h.

In terms of braking performance at 80 km/h on wet pavement, the tires with a remaining tread depth of 4 mm lose out to the new ones 7% of the braking distance.

Dry pavement

The braking distances provided by new and used winter tires on dry pavement are also different. However, counterintuitive as it might seem, the shorter braking distance was ensured by the tires with the smaller remaining tread depth. The reason is simple: the flattened blocks of the tread pattern get less deformed during the braking, and more of the tread gets in contact with the road. Nevertheless, an absolutely dry road is something that is not really common even in countries with mild winter climate, and this local victory cannot be considered to be a practical advantage!

Conclusions

This winter tire test proved that the remaining tread depth has an enormous influence on the performance of winter tires. This is equally applicable to summer and all-season tires. If you care about safe driving, you must have you tires replaced well before the tread depth wears down to three or four millimeters.

Also, be warned: using winter tires in the summertime is a bad idea. Due to the specifics of the tread pattern and the rubber compound that they are made of, their braking distance is considerably larger than the summer tires.