What tread actually does
The grooves in a tire are channels. At speed on a wet road they push water out from under the contact patch; in slush and snow they pack and release snow, which grips other snow. As the grooves get shallower, they hold less water and less snow, and the tire starts to ride on top of it instead of cutting through.
New vs worn
Most new passenger tires start at roughly 10/32 to 12/32 in (about 8–9.5 mm). Winter tires and light-truck tires often start deeper. Tread wears gradually, but grip in snow and heavy rain falls off well before the tire looks bald.
The numbers that matter
Three figures to remember:
- 5/32 in (about 4 mm): recommended replacement point for winter tires.
- 4/32 in (about 3 mm): a sensible replacement point for all-season tires if you drive in heavy rain.
- 1.5 mm: the legal minimum in Ontario. Built-in wear bars sit at about 2/32 in (1.6 mm), so a tire flush with them is at the limit.
How to measure
A tread depth gauge gives a real number. Push the probe into a main groove, not onto a wear bar, and read it in 32nds or millimetres. Measure the inner, centre and outer grooves on every tire. Wear bars (small raised bridges across the grooves) tell you when a tire has worn down to about 2/32 in (1.6 mm), right at the edge of the legal minimum. A coin can give a rough indication, but it isn't precise enough for a replacement decision.
Read the pattern, not just the number
If the inner edge is much lower than the outer edge, or the centre is worn more than the shoulders, the tire is wearing unevenly. That points to pressure, wheel alignment or balance, and fixing the cause protects the next set. See why tires wear unevenly.
When we measure it
We note the tread depth at every seasonal changeover and when we store your tires, so you hear about a worn set a season ahead rather than on the first snowy morning. When it's time to replace, our brands and prices page explains the options.
