Is tread always measured at the wear bars? I try to never miss your articles.
Thanks – Al M.
The short answer is no. The easiest way for any driver to test the tread depth is to find a spot that looks the most worn and measure using a coin, ideally a toonie. What you are testing is the depth of the groove – less than two millimetres for a summer tire or an all-season tire in the summer, or less than four millimetres on a winter tire or an all-season or all-weather tire in the winter – and it’s time to go tire shopping.
Here is what Ontario’s Vehicle Inspection Standard Handbook says about how technicians should check: “Inspect the tire tread to locate the area where the depth is at its minimum. Measure it at a major groove using a suitable tread-depth gauge.”
Also, per regulations, we are to reject a tire from passing a safety when the tread depth is less than two millimetres at any point. Regulations across Canada are similar.
Ontario’s handbook continues on to say, “Do not measure tread depth on a wear bar.”
The tire wear bars are small parts of rubber in the grooves. Basically, when the tread wears to the point that it is at the same depth as the wear bars, the tread is at the end of its life.
The actual tire tread wear bar is the manufacturer’s visual guide to when they consider their tire to be worn out. As there is no industry standard, the bar height varies and some tires do not have them. These days, most new tires have them and because the larger market for tire manufacturers is south of our border, the height is usually 2/32 of an inch (1.58 millimetres).
However, just because a tire is above two millimetres doesn’t mean it shouldn’t be replaced sooner rather than later.
Referring specifically to all-season tires, one that has three millimetres remaining will still be legal in most provinces. But make no mistake, this tire will be horrible in the rain, especially as it gets colder. The chances of hydroplaning increase as a tire approaches the last usable part of its life. I can understand that if you must make a tire replacement decision going into the summer, it will be tougher with a tire at three millimetres remaining as there is still some life left in it. But living with a three-millimetre remaining all-season tire as we enter the colder months makes zero sense to me.
Speaking specifically about winter tires, Transport Canada states to not use ones that are at or less than four millimetres of tread depth remaining. The same goes for all-weather tires. I agree. A four-millimetre remaining winter tire is almost useless in the snow. It will pass a safety inspection, but I wouldn’t want to be driving on them in a winter storm.
A tread depth measurement tool (which is what we use in the shop) is the best for the job, but thinking like a Canadian you may have a Canadian-made tool right in your pocket or handbag – a toonie. Find the shallowest, most worn part of the tire and insert the toonie between the treads. If the tread approaches the bear’s paw, the tire is probably brand new. If it covers the silver part completely, that is approximately 50 per cent, and if it’s halfway into the lower letters of the outside silver part of the coin, it’s time to go tire shopping. Using a toonie like this works for all types of tires.
I also need to discuss tire dating and ultraviolet (UV) cracking. All tires have a DOT or Tire Identification Number so that manufacturers can track defects and recalls. Find the 12-digit DOT number located on the sidewall and look at the last four digits. These represent the week and year the tire was manufactured. Most manufacturers declare that the usable life of a tire is about six years from installation date, not the manufacture date. One can only hope that the tire was installed within a year of being shipped from the factory. I cannot tell you how many times customers bring a car in for service with a set of recently purchased used tires that are at least 10 years old, complaining about new noises and poor traction. The rubber becomes far less pliable as it ages and traction results vary wildly on an aged tire.
UV cracking is the final point. As a tire ages, UV light, chemicals picked up from the road and harsh weather cause the compounds in the tire to break down, resulting in small cracks. These cracks are referred to as UV degradation and are a safety hazard when they are three millimetres deep or greater.
Lou Trottier is owner-operator of All About Imports in Mississauga. Have a question about maintenance and repair? E-mail globedrive@globeandmail.com, placing “Lou’s Garage” in the subject line.
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