Using Your Cents Wisely: What is the Penny Tire Test?

Understanding the Importance of Tire Tread Depth

Maintaining proper tread depth ensures that your tires perform at the highest level, especially on wet and snowy roads. Also, tires with almost no tread depth are not legal, which means that you can get a ticket for using them. But let's dive in.

1. Why Tread Depth Matters

A tire with no tread depth would produce the highest grip on dry roads because it creates the largest contact patch with the road. For that reason, racing cars ride on slicks when the conditions allow.

However, in the real world, it rains throughout the year and snows in the winter. On top of that, public roads are full of debris. To combat these conditions, tiremakers put cuts on the tire's surface, i.e., the tread.

These cuts ensure that water is channeled from the point of contact between the tire and the road, thus increasing traction. If a tire can't channel the water out of the contact patch, it will hydroplane, i.e., float over the surface. When that happens, the driver won't be able to control the vehicle, as there will be no traction on offer. Snow is even worse, as it requires small cuts (sipes) on the surface to act as biting edges.

Furthermore, the tread can also clean the contact patch from debris on the road, which improves traction and also protects the rubber compound. A race car tire doesn't need to do that because tracks are meticulously cleaned before and after each race.

The bottom line is that the tread pattern ensures that the tire works at least in dry and wet conditions (summer tires). On all-season and winter tires, the tread is more aggressive, making it usable on snow and ice. Without sufficient tread depth, your vehicle will be unsafe to drive on the road because of longer stopping distances in rainy and snowy conditions, lower lateral grip, and general instability.

In addition to that, although it's counterintuitive, lower tread depth also increases rolling resistance, which leads to higher fuel consumption.

2. Minimum Legal Tread Depth

The minimum legal tread depth varies by location, but a common standard in many regions, including the United States, is 2/32 inches (1.6 mm). However, in most countries, this limit only applies to the spring, summer, and fall. For wintry conditions, the minimum required tread depth in many countries in the world is 5/32 inches (4 mm).

However, the legal limit is only the bare minimum, and it doesn't mean that the tire will work optimally at that tread depth. Namely, a tire with 2/32 inches of tread depth will provide some hydroplaning resistance, but if you want to be safe while driving, I (and many experts) recommend replacing the tires sooner.

For instance, I always replace my tires when they reach 4/32 inches (3.2 mm) of tread depth. That is the point when I feel that the tire still works safely in wet conditions. Going well below that exponentially increases hydroplaning, meaning I'll need to drive much more slowly when it rains. To measure that, I use the quarter tire test instead – but more on that later.

What is the Penny Tire Test?

The Penny Tire Test is a simple yet effective way to measure the tread depth of your tires at home using nothing more than a penny. However, this test only applies to US citizens, as other countries have different coins. But don't worry, because I'll also cover the UK, Canada, and Europe using coins readily available there.

1. History and Concept of the Penny Tire Test

The Penny Tire Test is a method that has been around for years, allowing drivers to gauge whether their tire tread depth is safe and legal.

The concept is based on the physical features of a penny—specifically, the distance from the edge of a penny to the top of Abraham Lincoln's head, which is approximately 2/32 inch (1.6mm), the minimum legal tread depth in many places.

It's a quick and easy method that anyone can use not only to measure the tread depth but also to check for uneven wear.

2. How to Perform the Penny Tire Test

• In the US

Find a penny and insert it into one of the grooves of the tire, making sure that Abraham Lincoln's head faces you and points down. This is important, as the head will show you how much tread depth is left on the tire.

Namely, if any part of Lincoln's head is covered by the tread, you have at least the minimum legal tread depth. That means that the distance from the bottom of the coin is higher than 2/32 inches (1.6 mm), which is the legal minimum.

However, if you can see all of Lincoln's head, your tread depth is less than 2/32 inches (1.6mm), and it's time to replace your tires.

• In Canada

Canadian citizens can use the nickel to perform the penny tire test. Just like with the US penny, ensure that Queen Elizabeth's head is facing toward you and is upside down. If the tread covers the crown or face of the queen, it means you have enough tread depth left on your tires.

However, if you can see the top of the crown, it means that the tread depth reached 2/32 inches (1.6 mm), which is the lowest legal limit.

• In the UK

Citizens in the United Kingdom can use the 20p coin, which has an outer band that is roughly 2 mm thick. This doesn't directly correlate to the minimum legal tread depth in the UK, but it's close enough for most purposes.

Like with previous tests, insert the coin in the grooves of the tire. If the outer band of the coin is visible when inserted into the tread, the tire may be below the legal limit, and you should start looking for a replacement.

• In Europe

In Europe, the minimum legal tread depth is 1.6 millimeters across the central three-quarters of the tire and all around its circumference.

The outer band of a 1 Euro coin is approximately 2mm thick, which can be used to roughly gauge if a tire's tread depth is above or below the minimum.

Just like with the UK's 20p coin, inert the 1 Euro coin inside the grooves of the tire. If the outer band is not visible, it means there is enough tread depth left. However, if it is visible, the tires should probably be replaced.

Interpreting Penny Test Results

Understanding the results of the penny tire test can help you make informed decisions about your tire maintenance and safety.

If only a part of Lincoln's head is still visible, it doesn't mean your tires are safe to use. Sure, they will be legal, meaning you won't get a ticket, but their overall performance in the rain will be worse than when they were new. That is especially true under braking, where the stopping distances will be much longer than on a tire with sufficient tread depth left.

Since safety should be the number one priority when driving, I strongly recommend replacing the tires sooner than the legal minimum. By doing that, you are ensuring your safety, but also the safety of other traffic participants. Not to mention, a tire with low tread depth is more likely to cause an accident, which can be costlier than buying a new set of tires.

Limitations of the Penny Tire Test

The penny tire test is definitely useful in a pinch, but it does have its limitations, mainly because it's only useful to show you when the tire reaches the minimum tread depth. Let's have a closer look.

1. Factors the Penny Test Might Miss

The penny tire test only provides a rough estimate of tread depth and doesn't measure it accurately. It's possible for your tires to pass the test but still be close to the minimum legal tread depth. This gives a false sense of security to drivers, who would think that the tires are as safe as they come.

Also, while you can measure the tread depth in different grooves of the tire with the penny test, it won't alert you to small differences. Thus, it can't be really used for measuring uneven wear, especially if the issue is in its infancy. This is crucial, as catching uneven wear late will mean that you need to replace the tire.

Furthermore, the penny test can't identify signs of damage like cuts, punctures, bulges, or sidewall deterioration, which can all make tires unsafe even if the tread depth is adequate.

2. The Importance of Regular Professional Inspections

Nothing will replace a professional technician when it comes to inspecting a tire. An expert's eye can see what even a machine can't and find small issues, like cracks and cuts, that can develop into serious problems down the line.

Notably, a trained mechanic can accurately measure tread depth across the entire tire surface, check for uneven wear or damage, and ensure your tires are properly inflated and aligned. Also, he will check for bulges and any problems with the internal construction of the tire.

Alternatives to the Penny Tire Test

The penny tire test is very convenient and the only method of measuring tread depth known to most drivers. However, you can do better than that – and you don't need any tools!

1. The Quarter Tire Test

This is the test I am sharing with my friends when they ask me how to measure tread depth. Instead of the penny tire test, I recommend the quarter test.

It works similarly to the Penny Test but uses a quarter instead. Because the distance from the coin's edge to George Washington's head is approximately 4/32 inch (about 3.2mm), this test can help you identify when the tread depth is getting low but before it reaches the minimum legal limit.

As I already said, your tires will have severe hydroplaning issues even before they reach the minimum legal limit. In my testing throughout the years, a tire with 4/32 inch (3.2 mm) of tread depth has the bare minimum hydroplaning resistance I would consider safe for driving in rainy conditions. Thus, I always replace my tires when they reach that point.

Still, while the quarter tire test is better than the penny test, it still can't be used to catch uneven tire wear while it's in its infancy.

2. Using a Tread Depth Gauge

A tread depth gauge is my favorite way of measuring tread depth. I always keep one in my trunk since it's tiny, and nowadays, they are pretty cheap. Not to mention, tread depth gauges are far more precise than any test that involves coins.

To use one, you simply insert the probe into a groove in the tread and read the depth off the scale. Most tread depth gauges today are digital, which means that they provide very accurate readings down to a millimeter. As a result, you can also find irregularities like uneven wear when they are in their infancy.

Furthermore, the tread depth gauge is great for winter tires. As I mentioned, you will need at least 5/32 inches (4 mm) for your tires to be legal during the winter, and there is no coin test that can measure that for you.

3. Commercial Tire Wear Indicators

Every modern passenger car tire today is equipped with wear indicators. These are narrow rubber bars that sit into the tire's grooves and protrude 2/32 inches (1.6 mm) from the bottom. Thus, when the tread reaches its lowest legal depth, the tire's tread will be flush with the wear indicators.

These indicators are a good way to quickly check whether your tires are legal. However, they have even more limitations than the penny tire test. Namely, the wear indicators are made from the same material as the tread and will wear down at the same rate (once they start touching the road's surface). Thus, if you haven't checked your tires for a while, and the tread went below its minimum legal limit, the wear bars will still be flush with the tread, yet its depth will be lower than the legal limit.

Some manufacturers include more clever solutions. For instance, Continental puts the letters D (dry), W (wet), and S (snow) on some of its models. These letters wear down at different rates, and when one of them is invisible, it means the tire isn't safe for driving in those conditions. For instance, once the letter S wears down, the tire won't be safe to drive on snow-covered roads.

Conclusion

Measuring the tread depth on the tires is a very important practice but not the only method of showing the health of the tire. Although the tread depth is important for driving in rainy and snowy conditions, other factors, like the rubber compound, play a crucial role in ensuring the highest traction in all conditions, including dry, wet, and snowy.

For instance, dry rot, which is a process that causes the rubber compound to decay over time, can greatly reduce the tire's performance. For that reason, tiremakers and expert technicians recommend replacing the tires after 6-8 years, even if they have sufficient tread depth left. However, some external factors, like the sun's UV rays, can accelerate dry rot.

Furthermore, things like cracks, cuts, punctures, and bulges can cause blowouts, which are significant safety issue. Also, they could cause tread separation, which is another dangerous occurrence that can seriously destabilize any vehicle.

For all those reasons, you should ask the technician to inspect the tires at each oil change. That way, you will always know whether they have enough tread depth left or whether there are other irregularities.