Tyre wear calculation
Discussion
If you had tyres that were notionally 7mm from new and that across 2/3 of the tyre, there was still 7mm of tread left and the inside was down to 6mm, what would you say was the percentage of wear?
Would the calculation be across the whole tyre of only across the worn section?
Hopefully, the above makes sense!
Would the calculation be across the whole tyre of only across the worn section?
Hopefully, the above makes sense!
If you're thinking about the legality of a tyre, the minimum is 1.6mm across the central three-quarters of the tyre. So if the 'inside' band that was down to 6mm is more than one-eighth of the width of the tyre, then the tyre has 6mm of tread remaining. If it's still min 7mm across the central 3/4s then it's still 7mm legally.
If you're thinking from a safety point of view I would generally take a model that the total volume of inter-tread gap will be available to dispel the water. So from that angle I'd take an average across the whole width. So if it was 6mm on the inside edge graduating to 7mm by one-third of the way in, and then maintaining that 7mm to the outside edge, the average tread depth would be 6.83mm (6 and 5/6ths mm). This probably best predicts aquaplaning performance.
If you're thinking from a safety point of view I would generally take a model that the total volume of inter-tread gap will be available to dispel the water. So from that angle I'd take an average across the whole width. So if it was 6mm on the inside edge graduating to 7mm by one-third of the way in, and then maintaining that 7mm to the outside edge, the average tread depth would be 6.83mm (6 and 5/6ths mm). This probably best predicts aquaplaning performance.
pincher said:
My reason for asking is that one of my cars had a service the other day and they do one of those health check videos - they told me that the rear tyres, which were new two weeks and (max) 250 miles previously, were 7mm, 7mm, 6mm (outer to inner) and 31% worn 
If they assume the tyres came with 8mm tread and the legal minimum is 1.6mm then that gives 6.4mm of useable tread.As your lowest reading of tread depth was 6mm this gives 2mm of wear out of 6.4mm = 31%
This does highlight how difficult it is for tyre buyers to learn how much tread a specific new tyre comes with, as above i suspect the garage assumed the tyres came with 8 or 9mm of tread when new.
On tyre makers websites and on tyre sales sites it should be a requirement to display the tread depth, some sites have this info on some but not all tyres, if i had this info no way would i buy a set with only 7mm when new.
Nokian tyres have a useful feature, tread depth remaining in mm is in the tread moulding, as the tread wears down the new depth is revealed, simple.
On tyre makers websites and on tyre sales sites it should be a requirement to display the tread depth, some sites have this info on some but not all tyres, if i had this info no way would i buy a set with only 7mm when new.
Nokian tyres have a useful feature, tread depth remaining in mm is in the tread moulding, as the tread wears down the new depth is revealed, simple.
Scrump said:
pincher said:
My reason for asking is that one of my cars had a service the other day and they do one of those health check videos - they told me that the rear tyres, which were new two weeks and (max) 250 miles previously, were 7mm, 7mm, 6mm (outer to inner) and 31% worn
If they assume the tyres came with 8mm tread and the legal minimum is 1.6mm then that gives 6.4mm of useable tread.As your lowest reading of tread depth was 6mm this gives 2mm of wear out of 6.4mm = 31%
pincher said:
My reason for asking is that one of my cars had a service the other day and they do one of those health check videos - they told me that the rear tyres, which were new two weeks and (max) 250 miles previously, were 7mm, 7mm, 6mm (outer to inner) and 31% worn
Have the new tyres really lost 1mm of tread in just 250 miles? If it really is wear, I would get the alignment checked/adjusted asap.Pica-Pica said:
Scrump said:
pincher said:
My reason for asking is that one of my cars had a service the other day and they do one of those health check videos - they told me that the rear tyres, which were new two weeks and (max) 250 miles previously, were 7mm, 7mm, 6mm (outer to inner) and 31% worn
If they assume the tyres came with 8mm tread and the legal minimum is 1.6mm then that gives 6.4mm of useable tread.As your lowest reading of tread depth was 6mm this gives 2mm of wear out of 6.4mm = 31%
It's no big deal in the scheme of things, just everso slightly irritating.
That’s the thing - if they hadn’t been so new, I probably wouldn’t have taken anywhere near as much notice. As I said, they were put on two weeks earlier and other than a bit of town driving, I drove from Essex to Pangboirne to Marlow and back to Essex - just think it was a bit of a nonsense that they were almost a third worn.
I emailed the (main) dealer back with a suggestion that they could just report the depths as they find them, rather than trying to make a %age calculation based on assumptions - probably won’t hear back but it made me feel all superior
Edit - oddly enough, mine were PS4S as well
I emailed the (main) dealer back with a suggestion that they could just report the depths as they find them, rather than trying to make a %age calculation based on assumptions - probably won’t hear back but it made me feel all superior
Edit - oddly enough, mine were PS4S as well
Edited by pincher on Saturday 27th August 06:00
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