In general, how to assess the severity of a cut on the side of a tyre? How to tell if it safe to drive?

Note that I realise there are a few related questions, but they are asking about a particular cut (with photos) which isn't extremely useful for other people. This question is about what to look for in general when assessing the road-worthiness of a tyre with a cut.


I am about to buy a small car (ford fiesta, in the UK) and I noticed a cut of the side of one of the tyres. I did not take a photo of the cut. I want to know what to check for when I go back.

The dealer refuses to change the tyre(s) and argues that since the car passed AA inspection last week, the tyres are safe. I am still concerned as the only tyre-related item in the AA report is tread depth (4-5mm in my case) but there is no mention that they checked for cuts/bulges. Maybe I am worrying too much.

Extract from the AA inspection report: Extract of AA inspection


In the UK, tyres are subject to a particular check for cuts in the MOT test, for depth as well as length. The relevant part of the testers' manual says (inter alia):

Reason for rejection

a. A tyre has a cut the length of which is in excess of 25 mm or 10% of section width, whichever is greater, deep enough to reach the ply or cords

b. a tyre has:
• a lump, bulge or tear caused by separation or partial failure of its structure. This includes any lifting of the tread rubber
• any of its ply or cord exposed

It's also acceptable to be thorough in checking how deep a cut is:

Note: It is permissible to check for exposed ply or cord by using a blunt instrument to open a cut, taking care not to cause further damage.

You can either measure the width of the tyre, or take it from the nominal size.

This is an objective test. If it's an MOT failure, point that out. However, even the objective test is only instantaneous and is no indication of whether the tyre will actually fail or not, or when that might occur, if it does.

Tyres are covered by the AA check and I think I would have expected a "detailed report" to include a mention of a cut, even if they say it's not an MOT failure. If it doesn't mention a cut at all, it seems likely to me to have occurred after the inspection was carried out. What else has happened since the inspection?

Basically, if you are worried about a tyre you will need to get it changed: it's obviously best if you can get the dealer to do that (or to give you a discount for the tyre of your choice if you decide to buy the car with the intention of changing the tyre). If he won't do that, the price is going up by the cost of the tyre. Your and your passengers' life is certainly worth the cost of a tyre: is the car worth the total amount?

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  • Thank you for your answer Andrew - very helpful. I saw the car before the inspection so the cut was most definitely present when the engineer checked the car. I added the relevant bit of the "128-point vehicle inspection" to the post. It does not mention the cut - should I understand there was nothing major to report? The MOT was passed in June so fairly recent as well; although this was before my visit so I don't know if the cut was there or not. In any case thank you for the advice, I'll be sure to have the tyre checked by a professional if I end up buying the car. – asac Jul 12 '17 at 18:50
  • As I wrote, I would expect a detailed report to include mention of the cut, even if it isn't an MOT failure, because it proves the engineer saw it. For myself, its absence casts doubt on the report. You will need to make your own assessment, of course. Even if my penultimate paragraph isn't entirely relevant, the last paragraph still is. – Andrew Leach Jul 12 '17 at 18:55
  • Actually, "it's obviously best for dealer to do that" isn't always true as they may get entirely the wrong tyre merely to fix the cut. That's less likely to happen with a Fiesta but some cars require a specific manufacturer/type or combination of tyres: the dealer may be a cheapskate. And if he's prepared to sell a car with a cut, that might well be the case. – Andrew Leach Jul 13 '17 at 8:24

Considering the sidewall of the tire bears a great deal of stress and strain in its function, it is a good thing you are paying attention to this cut.

In general, there is no simply answer. Each cut is an individual case as to whether it is catastrophic damage or not. Some things that determine this are the depth of the cut, position of the cut, and the age of the tire. If there is no bulging or puckering near the cut, it might be alright.

If possible, take the car to a tire professional and have the tire evaluated. A simple rule of thumb with tires, "when in doubt, swap it out". No sense in taking a chance of having a blow out that might cause an accident.

If the dealer wants to sell you the car, they will change the tire.

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  • Thank you for your answer. I understand that each cut is an individual case, but how do you diagnose the case? You touched on this by mentioning depth, position, age of the tyre ; could you please elaborate? E.g. what positions are worth than others? How to spot an unsafe depth? Etc :) Thanks! – asac Jul 11 '17 at 18:31
  • As above no simple answer. I would suggest that if the cut is less than 1mm deep then you may be ok, after that you need a professional opinion... – Solar Mike Jul 11 '17 at 19:26
  • Sorry. There isn't a simple way to transfer experience thru the Internet. Besides, I know nothing about the tire you are concerned about, other than it is most likely round and black. :-) – CharlieRB Jul 11 '17 at 23:21

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