Real tread wear on a tyre
It looks like a simple question: what is the wear (in per cent) on a tyre with a tread block height of 4 mm? In cases like this sellers usually write a “modest” 30%, but what is the truth?
How do you work out the real tread wear on a tyre?
What could be simpler, you ask? Divide the actual remaining tread depth by its starting depth, and we get…. Except we do not get anything of the sort, in reality, even though sellers of part-worn tyres (at best) work out wear exactly like that. We do not, mainly because that formula would only make sense if you could actually run a tyre right down to “zero”, and that is impossible.
Tyre makers, as a rule, put tread wear indicators in the tread at a height of 1.6 mm; reaching them already officially means the tyre is completely unfit for further use. Ukraine’s traffic regulations prohibit driving on tyres with less than 2 mm of tread remaining. Driving on a tyre with less tread than that is not only illegal, it is extremely dangerous. And if we are talking about winter tyres, the real “zero” is even higher: 3-4 mm, depending on the tread type and how the tyre is used.
So, to work out a tyre’s real wear, you should use this formula:
(Hn-Hf)
Z = ---------------------- *100%
(Hn-Hmin)
where:
Z is the tyre’s real wear as a percentage (note: wear, not what is left!). The percentage left is 100 - Z
Hn is the tread depth of a new tyre of this model and size.
Hf is the actual tread depth (the lowest of all the measurements taken on both tyres you plan to fit to one axle)
Hmin is the minimum permitted tread depth for this tyre.
It will not always be possible to find out what the tread depth was on a new tyre of the model you need (Hn), so, based on the differences between tyre types in new tread depth, when working out wear you should use the following average figures:
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For winter tyres with a Nordic tread pattern (off-road-capable tyres, including studded ones), Hn = 10 mm.
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For winter tyres with a classic or asymmetric tread, Hn = 9 mm.
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For high-speed winter tyres (with a tread pattern resembling a summer one), Hn = 7 mm.
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For summer tyres with a classic tread and increased resistance to aquaplaning (rain tyres), Hn = 8 mm
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For high-speed summer tyres, Hn = 7 mm
To work out the actual tread depth (Hf), carefully measure both tyres on one axle and take the lowest tread-depth figure of all the ones you get. (More on uneven tread wear.)
Things are a little more complicated with the minimum permitted tread depth (Hmin). The critical figure (the absolute zero) under the traffic regulations is 2 mm, but that figure only really applies to summer tyres, and not even to all of them. A winter tyre, on the other hand, loses the vast majority of its characteristics as early as around 4 mm (on average). So your real minimum will sit somewhere in this range: from 2 to 4 mm, depending on the tyre type, the season and your own personal conditions of use. It is worth remembering that you are the one buying the tyre, for yourself, so you will have to work out wear with a certain amount of subjectivity.
It is also important to understand that the method for working out a tyre’s real wear given above is meant purely to help you decide for yourself whether buying a particular part-worn tyre makes sense, not for making any claims against the seller - you will most likely never manage to convince them that wear is worked out the way it is written here.