Michelin Pilot Sports - tread depth question

My 2004 996 4S is still fitted with the original Michelin Pilot Sport 295/30 ZR18 N1 tyres. One rear tyre is, I think, approaching the wear limit, but I am very confused as to whether or not it is still legal. In the centre region, there is about 4-5mm of tread - well over the 1.6mm minimum. However, at the edges there is about 1mm, definitely less than 1.6mm. Nevertheless, tread is still clearly visible over the entire circumference. I know that I need 1.6mm over the centre area and some visible tread over the entire width. Looking closely at the tyre, Michelin have designed it so that the centre tread area is in fact deeper than the side edges, where the grooves do not extend as deeply into the tyre. The difference seems to be about 2mm (ish), suggesting that the tyre is designed so that when the edges have no tread, the central area has the legal minimum. So the whole tyre "runs out" of tread at the same time, assuming even wear. Now, IIRC the law prescribes that that centre area needs to be 75% of the tyre width. This is where I get confused. Michelin's centre area is 5.5 inches, exactly 50% of the (11 inch) tyre width. On that basis, the tyre is no longer legal to drive because the 1.6mm minimum tread does not extend over 75% of the width of the tyre. However, Michelin have put wear bars are in the centre area, which would suggest that this is the relevant part for testing. Their website advises me to look at these. On this basis, the tyre is still legal. Perhaps there is some exception for seriously fat tyres? Or maybe my understanding of the 75% requirement is out of date? I've contacted Michelin and will let you all know what they say, but all thoughts would be appreciated.

This is the legal requirement Tread Depth The legal minimum tread depth for cars and light trailers (including caravans) up to 3500 kgs gross vehicle weight and/or 8 seated passenger vehicles including driver is: A minimum of 1.6mm in a continuous band throughout the central three-quarters of the tread width, throughout the whole of the circumference (see illustration below). Any vehicle with a gross vehicle weight (GVW) or gross train weight (GTW) over 3500kgs or a motorcycle above 50cc must, either: The grooves of the tread pattern must have a depth of a least 1 mm throughout a continuous band measuring at least three-quarters of the breadth of the tread and round the entire outer circumference of the tyre, OR if the grooves of the original tread pattern of the tyre do not extend beyond three-quarters of the breadth of the tread (i.e. common with motorcycle tyres) any groove of the original pattern must have a minimum depth of at least 1 mm.

Thanks Geoff - found that site over the weekend. It does look like the tyres need replacement, but I'm confused by Michelin's placement of the wear bars. If the tyre wore evenly then by the time they surfaced, it would seemingly be illegal. Odd.

Michael I sympathise. I too went to the extent of ordering new rears for my 996 only for the fitter to (very kindly) advise me they were nowhere near the legal limit and I had quite a bit of life left in them. He kept them in stock for nearly 3 months for me. But they looked 'illegal' to me because of the edges. Neil

Michelin's reply: So my mistake is measuring the 75% as being of the tyre width, as opposed to the tread width. The latter seemingly excludes the fine pattern on the shoulders. On that basis, my tyres are legal and have quite some life left in them. All credit to Michelin. A quick and helpful reply to an issue that was worrying me (and keeping the car in the garage!). Pilot Sports are now the likely replacements when the time does come.

Don't forget that Porsche recommend 3mm minimum tread depth. Because my car is a toy and gets very little wet weather use, I work on a 2mm as a rule of thumb.

My gut feeling is that if that gave you better grip at the front than at the rear, then you may get into trouble.