When should I replace my tires?

While it’s easy to stick a Lincoln penny into the tread groove of a worn tire and use Abe’s image as a guide, we don’t think honest Abe necessarily knows what’s best for today’s drivers.
The Lincoln penny tread-depth test has long been promoted as a handy way to decide when it’s time to buy new tires. It’s based on the idea that you’re still driving on legal minimum tread depth when the top of Lincoln’s head is covered by the tread, and that a tire’s ability to grip the road isn’t affected by bad weather (rain, hail, sleet and snow). It’s not quite that simple.

Car tire tread design

Your ability to control your vehicle depends on the traction between your tires and the road. Tires don’t need complex tread patterns or a lot of tread depth to provide grip on dry roads. A practical example is the racing slicks used on standard-bodied race cars and open-wheel race cars that still deliver grip at speeds over 200 km per hour. However, tires do need tread patterns to generate traction and grip on wet, muddy and snowy roads.
A tread pattern is needed to channel water and mud away from between the tire and the road surface, and to create edges that can bite into snow. But that’s only half the story, because tread depth also plays a big role in how well that pattern can do its job.

The air your tires encounter at highway speeds can easily be compressed and pushed aside with relatively little effort. Liquids, however, are a different story. When water builds up on the road surface during heavy rain, the water depth, vehicle speed and vehicle weight, as well as the tread pattern and remaining tread depth of the tires, together determine when the car will start to lose grip. So it depends on multiple factors.

Wear

Obviously, the tread will wear down over the life of the tire and the volume of the tread grooves will decrease. Because this happens gradually, you may not notice it from one day to the next, but eventually there will come a point when you feel the car start to slide in snow, skid in the rain or simply take longer to come to a stop on wet roads. And that comes with all the associated risks.

Minimum tread depth

Tests have been carried out on vehicles fitted with a set of new tires and a set of worn tires (tread depth of 1.6 mm and around 3 mm) to see what the difference in braking distance is. The differences are huge and surprising. The recommendation is that if you’re worried about rain and wet roads, you should consider replacing your tires when they reach around 3 mm of remaining tread depth. Since water cannot be compressed, you need sufficient tread depth to allow rainwater to escape through the grooves in the tire. If the water can’t escape quickly enough, your tires will be forced to hydroplane on the water, causing a loss of traction and longer braking distances, and making the car almost impossible to control. In addition, if you’re concerned about snowy roads, you should consider replacing your tires when they reach about 4 mm of tread depth to maintain good mobility. You need more tread depth in snow because your tires have to compress the snow into their grooves and then release it as they roll on. If there isn’t enough tread depth, the “bites” of snow your tires can hold each revolution are reduced to tiny “nibbles”, and your vehicle’s traction and mobility will be compromised.

Although it may not seem like the best choice financially, it’s unwise to keep driving on your tires until they reach the minimum tread depth of 16 mm. For your own safety as well, it’s recommended to replace car tires when they have around 3 mm of tread remaining. Also read the articles “The best car tires” and “Replacing a car tire” so you know how it all works.

And don’t forget to regularly check and replace your tire valves. This isn’t something you immediately think about, but the valve is essential. Why? Read more about that here.