Your tires are the only part of the car that actually touches the road. Every bit of braking, steering and grip goes through four patches of rubber about the size of your hand. When the tread wears down, those patches can’t clear water fast enough, and stopping distances in the wet get noticeably longer, well before a tire looks “bald”.
The good news: checking takes five minutes and costs nothing. Here’s how to do it properly.
Why tread depth matters
The grooves in a tire are channels that push water out from under the contact patch. New passenger tires usually start with around 8–9 mm (10/32″ to 11/32″) of tread. As that depth drops, the channels hold less water, and the tire is more likely to aquaplane, riding on a film of water instead of the road. Grip on snow and slush falls off even faster, because the tread also needs depth to bite into it.
Three ways to measure
1. A tread depth gauge (best)
A basic gauge costs a few dollars at any parts store and gives you a real number. Push the pin into a main groove, press the base flat against the tread blocks and read the scale. Measure in three places across the tire (outer edge, centre, inner edge) and at two or three points around it. Use the lowest reading.
2. The tread wear indicator bars
Look inside the main grooves for small raised bars running across them. They sit at about 1.6 mm (2/32″). If the tread is level with those bars anywhere on the tire, it’s worn out. Many tires also mark where the bars are with a small triangle or “TWI” on the shoulder.
3. The coin test (quick check)
No gauge handy? Slide a coin into the groove and see how much of it disappears. In the US, the classic version uses a penny: put it in head-first, and if you can see the top of Lincoln’s head, you’re at roughly 2/32″ or less. A quarter does the same job at a better threshold: if the tread doesn’t reach Washington’s head, you’re under about 4/32″. Coin tests are rough, so treat a borderline result as a reason to get a proper measurement.
When to replace: legal minimum vs. safe minimum
In many places the legal minimum is 1.6 mm (2/32″), but the rules vary, so check what applies where you drive. Treat the legal minimum as the absolute floor, not the target. Independent testing has repeatedly shown wet braking getting markedly worse well before a tire reaches it. A sensible rule of thumb:
- Around 3 mm (4/32″): start shopping, especially if you drive a lot in rain.
- Around 4 mm (5/32″) for winter tires: below this, snow traction drops sharply.
- At the wear bars (1.6 mm / 2/32″): replace now.
Read the age code, too
Rubber ages even when the tread is deep. On the sidewall, find the code that starts with “DOT”. The last four digits are the week and year the tire was made: 2321 means the 23rd week of 2021. Many tire and vehicle makers recommend a professional inspection once tires reach about five or six years old, and replacement at around ten years regardless of tread. Check your tire maker’s guidance. This matters most on spares, trailers and low-mileage cars, where tires can look fine but be well past their best.
Uneven wear is a clue
How a tire wears tells you something about the car:
- Both edges worn, centre fine: usually under-inflation.
- Centre worn, edges fine: usually over-inflation.
- One edge worn: often an alignment problem.
- Scalloped or cupped patches: can point to worn shocks, struts or suspension parts.
Fixing the cause before fitting new tires stops you from wearing out the new set the same way.
Other signs a tire needs attention
- Bulges or bubbles on the sidewall (replace immediately; the internal structure is damaged).
- Deep cracks in the sidewall or between tread blocks.
- Exposed cords or steel belts.
- A new vibration at speed, or a tire that keeps losing pressure.
Make it a habit
Check pressures once a month with the tires cold, using the figure on the sticker inside the driver’s door (not the maximum on the sidewall). Check tread depth whenever you check pressure. Rotate the tires as your owner’s manual recommends, often every 8,000–12,000 km (5,000–7,500 miles), so they wear evenly. Five minutes a month is cheap insurance for the parts that do all the stopping.
This is general guidance. Always follow your vehicle and tire manufacturer’s recommendations and the rules where you drive. If in doubt, have a tire professional take a look.
