Tread depth is the single biggest factor in what a used tire is worth. It is also the easiest thing for a seller to exaggerate. A five-dollar gauge settles it in seconds.
Units: millimetres and 32nds
You will see tread depth in two units:
- Millimetres (mm), common in Canada.
- 32nds of an inch, common on US sites and gauges. New passenger tires usually start around 10/32 to 12/32 (roughly 8 to 9.5 mm), and many winter tires start deeper.
Most gauges show both.
How to measure
- Push the gauge's pin into a main groove, not on top of a wear bar.
- Press the flat shoulder of the gauge onto the tread blocks on either side.
- Read the number.
- Repeat on the inside, centre and outside of the tire, and at a few spots around it.
Use the lowest reading, not the best one.
The wear bars
Look in the main grooves for small raised bars running across them. These are tread wear indicators and sit at about 1.6 mm (2/32). When the tread is flush with the bars, the tire is worn out. A tire at the wear bars is not a bargain at any price.
How much is enough for winter?
Legal minimums are one thing. Grip on packed snow and slush is another. Winter tires lose a lot of their snow traction long before they reach the wear bars, because the deep grooves and sipes are what bite into snow and clear slush.
- Michelin Canada advises at least 3.2 mm for winter tires in Alberta, and British Columbia requires at least 3.5 mm on designated winter highways.
- For a used winter set you plan to run for a few seasons, we suggest looking for 6 mm or more. Below about 5 mm, you are buying one season, maybe two.
Many winter tires also have a second set of indicators on the tread, often a small snowflake or a number. When those wear away, the tire's winter performance is mostly gone, even if it still has some tread.
What uneven wear tells you
- Worn on both edges: run underinflated.
- Worn in the centre: run overinflated.
- Worn on one edge only: alignment problem on the old car.
- Cupped or scalloped patches: worn suspension or balance issues.
None of these mean the tire is unsafe on its own, but they lower the usable tread and the price you should pay. If the wear is badly uneven, pass.
Quick buying rule
Measure every tire. Use the lowest reading in the set. Compare the price against a new set with full tread. If you are paying more than about half the new price for half the tread, keep looking. When you bring a used set to our garage in Laurel, South Edmonton, we check the tread again before mounting.