How do I know when my tires need replacing?

Three numbers are worth knowing. They come from BC law and Transport Canada, and each one fits a different job.

• 1.5 mm. BC's legal floor for a car tire. Below it in two spots at least 38 cm apart, or with the wear bars touching the road in two grooves side by side, the tire is unfit for the road.
• 3.5 mm. The least tread a winter tire can have and still count on BC's signed winter routes.
• 4 mm. Transport Canada's line for snow. Tires worn close to it should stay off snow-covered roads.

For Kootenay winters, 4 mm is the number we watch. A tire at 4 mm is still legal. It has lost much of the depth that bites into snow and clears slush, though, and the passes will find that out first.

A tread depth gauge is cheap at most parts stores. Push the pin into a main groove and read the number. Check the inside, middle and outside of each tire, because wear is often uneven.

Most tires also have wear bars. These are small raised bridges across the bottom of the grooves, set at about 1.6 mm. When the tread is flush with the bars, the tire is done.

Some tires need replacing with plenty of tread left. Look for cuts or cracks deep enough to show the cords. Look for bulges or blisters on the sidewall. A bulge means the structure inside the tire has come apart, so that tire should be replaced before the next drive.

Forest Service Roads are hard on sidewalls. Sharp rock can slice a tire that still has deep tread. A frost heave or a pothole on Highway 3 can pinch a tire against the rim and leave a bulge.

Age counts too. Rubber hardens and cracks over time, even on a tire that sits in the shed. Find the code after the letters DOT on the sidewall. The last four digits give the week and year the tire was made. A code ending in 2219 means the 22nd week of 2019.

Michelin advises a yearly check by a tire professional once a tire has been in use for five years. It advises replacing tires ten years after they were made, whatever the tread looks like. That includes the spare.

A tire's wear pattern is a clue to what is going on under the car. Wear on one edge often points to alignment. Wear on both outer edges often means the tire ran low on air. A worn strip down the middle often means it ran too high. Scalloped dips around the tread can point to worn shocks or a tire out of balance.

Cold makes low pressure more likely. Transport Canada notes that a tire set in a warm garage will be underinflated below zero. Check pressures cold, using the label in the driver's door jamb. If your tires wear on one edge, our guide on when you need a wheel alignment explains the fix. A wheel alignment starts from $129.

If you are unsure, book a bay for a tire check. We measure all four, photograph anything we find, and send the photos with the quote. Tires are billed at cost plus 20 percent, and the supplier invoice is yours to see.