Tread depth is one of the few things on a car you can check properly yourself, in the driveway, in about five minutes. It's also one of the more useful checks, because the tread is what moves water and slush out from under the tire. Less depth means less room for that water to go.
What new tread looks like
Most new passenger tires start with roughly 8 to 9 mm of tread, about 10/32" to 11/32". Winter tires and light truck tires often start deeper. In North America tread is commonly measured in 32nds of an inch, so you'll see both units on gauges and on shop reports.
The tread wear bars
Every tire has small raised bars moulded across the bottom of the main grooves, at several spots around the tire. Some tires mark where they are with a small triangle or the letters TWI on the shoulder. The bars sit at about 1.6 mm (2/32").
When the tread is worn flush with the bars, the tire is finished. Replace it. But by that point it's been losing wet and snow grip for a long time. The wear bars mark the end, not the point where you should start shopping.
Using a tread depth gauge
A tread depth gauge is a small plunger-style tool sold at most auto parts and hardware stores, and it doesn't cost much. To use one:
- Push the pin all the way in.
- Set the flat base across a main groove, with the pin over the deepest part of the groove and not on a wear bar.
- Press the base flat against the tread blocks so the pin drops to the bottom of the groove.
- Read the scale. Most gauges show both millimetres and 32nds.
- Measure the inner, centre and outer grooves, then repeat at two or three spots around the tire.
Write the numbers down. The lowest reading is the one that counts, and the difference between the inner and outer grooves tells you how the tire is wearing.
When to replace
These are general guidelines that many tire makers use. Your tire maker's own recommendation may differ.
| Tire type | Consider replacing at | Why |
|---|---|---|
| Winter | About 4 mm (5/32") | Snow grip falls off as the grooves and sipes get shallow |
| All-season | About 3 mm (4/32") | Wet grip and resistance to hydroplaning drop noticeably |
| Any tire | 1.6 mm (2/32"), level with the wear bars | Worn out. Replace now |
Some winter tires have a second set of indicators, sometimes marked with a snowflake, that show when the tire is no longer much use in snow even though it still has tread.
About the coin trick
You'll see advice about pushing a coin into the tread and checking how much of it disappears. It's a rough way to tell a nearly bald tire from one with plenty left, and it beats not looking at all. It isn't a measurement. Coins vary, and the difference between 3 mm and 4 mm is exactly the range where you want a real number.
What uneven wear tells you
A healthy tire wears evenly across the tread. When it doesn't, the pattern usually points to a cause:
| Wear pattern | Likely cause | What to do |
|---|---|---|
| Both outer edges worn | Running underinflated | Check pressures monthly against the door placard |
| Centre worn more than the edges | Overinflated | Set pressure to the placard, not the sidewall maximum |
| One edge worn, inner or outer | Alignment out (camber or toe), or worn suspension parts | Alignment check and suspension inspection |
| Feathered ribs, smooth one way and sharp the other | Toe out of spec | Alignment |
| Cupped or scalloped patches | Worn shocks or struts, or a wheel out of balance | Suspension check and balance |
| One flat spot | A hard skid with locked brakes | Have it inspected. A deep flat spot can cause a vibration that won't balance out |
| Fronts wearing much faster than rears | Normal on front-wheel drive cars, made worse by skipped rotations | Rotate on the schedule in your owner's manual |
Inner-edge wear is the one that catches people. You can't see it standing beside the car, and the outer edge can look fine while the inside is worn down to the belts. Run your hand along the inner edge of the tread, carefully, since worn tires can have exposed wire, or have a look next time the car is up on a hoist.
Check twice a year
Before winter and before summer are the natural times. If we're doing your seasonal changeover, ask for the tread readings. With the wheels off the car it takes seconds, and you'll know a season ahead if new tires need to go in the budget.
