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Nokian winter tire tread question

Lui's Knob

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IIRC, it's the tread depth (as mentioned) in millimeters. When the number 4 shows, it means you should replace the tire soon. Sounds like you got hosed on the purchase ...


Dangers of aquaplaning

When driving on a dry road in the summertime, the driver hardly has to shed a thought for the tyres. When a thunderstorm breaks or a persistent summer rain fills the grooves in the road, it is quite a different ballgame. The danger of aquaplaning can be felt in the steering: it is difficult to control the car, especially if the tyres are in poor condition or worn out. The best way to prevent aquaplaning is to use new tyres. Even new tyres do not completely eliminate the risk of aquaplaning, but it is possible to control the car as long as you adjust the driving speed to the conditions.

Lost feel

From the viewpoint of traffic safety, it is important to have tyres with the proper groove depth and properties to suit the weather conditions. When there is plenty of water on the road and the driving speed exceeds a certain limit, the tread pattern of the tyre no longer pushes aside the water from underneath the tyre. The feel between the tyre and the road will be lost, and so will the grip.

When there is less than four millimetres of tread in the tyres, the tyres’ wet grip and aquaplaning properties essentially deteriorate; the risk of aquaplaning, in particular, greatly increases. Furthermore, the breaking distance is longer and the car will skid easier.

Tests conducted by Nokian Tyres show that with a worn-out tyre (tread less than 1.6 mm, approximately 5 mm of water on the road), aquaplaning will occur when driving in a curve at the speed of 76 km/h, whereas the aquaplaning speed for new tyres is 96 km/h.

Lost contact area

When the driving speed increases and the tyres wear out, the contact area between the tyre and the road is dramatically reduced. The figure shows the size of the contact area of a summer tyre with various groove depths, with a water thickness of three millimetres and driving speed of 75 km/h. The contact area of a vehicle with worn-out tyres, thread 1.6 mm, is only 16 per cent when compared to a stationary vehicle.

Contact area

8 mm


74 %

4 mm


58 %

1,6 mm



Contact area comparison with various groove depths of a summer tyre. Driving speed 75 km/h, 3 mm of water on the road. (Tyre Specialists of Finland 2007)

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Found my answer (back tires are faintly faded 6 to clear 4 so it'll still have 1 season of life left) got it for remarkably cheap so can't complain....

The DSI driving safety indicator displays the remaining tread in millimeters, and tells you when you should replace your tires. <b>It is recommended that the tire be replaced once the "4" has worn away. On some tires, DSI also includes a rain drop symbol. When this has worn away, the tires' hydroplaning capabilities are no longer optimal, indicating it is time for replacement. Once the "2" has worn away, the tires are no longer safe to drive and should be replaced as soon as possible.

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