We all want to get the most out of the money we put into our vehicles, but inevitably, even the most long-lived tires and rubber compounds wear down from driving. As tread wears away, both conventional knowledge and prior testing says that braking performance on wet roads drops dramatically, increasing stopping distances significantly. Tires are legally "worn out" in most States when they reach 2/32" of remaining tread depth, indicated by Tread Wear Indicator bars running from one side of the tread design to the other. Keeping enough tread on your tires is about more than just about legal requirements: it's about how quickly you can stop when it counts.
Knowing just how much of a difference a worn tire can make might be worth a second thought before taking them all the way down to the legal limit. As you'll see, even the difference between 4/32" and 2/32" can be the difference between stopping at an intersection, or on the other side of it.
Water on the road creates a barrier between the tire itself and the road's surface; it interferes with the basic function of a tire's ability to create traction, or more importantly, safely stop in a panic braking (emergency) situation. Water can be displaced, to an extent, but even a thin layer can reduce a tire's ability to grip the road, even at full tread depth.
Grooves molded into a tire's tread work to channel that water away from the contact patch, allowing the tire to sustain grip. However, as those grooves grow more shallow through regular use, they lose much of their capacity to evacuate water. When that happens, tires begin to hydroplane – riding up on a film of water – and braking distances skyrocket, even at moderate speeds.
In order to illustrate this, Tire Rack set out to show how critical proper tread depth is for safe braking, especially in rainy conditions.
Tire Rack compared stopping distances from 60 mph with three tread depths:
| Tread Depth Remaining | Braking Performance from 60mph | Total Distance to Stop | Speed When Passing Baseline |
|---|---|---|---|
| New (1 1/32") | Baseline stopping distance | 205 ft. | 0mph |
| Worn (4/32") | Requires 65 additional feet to stop over baseline | 270 ft. | 35mph |
| Barely legal (2/32") | Requires 195 additional feet to stop over baseline | 400 ft. | 50mph |
| Tread Depth Remaining | Braking Performance from 60mph | Total Distance to Stop | Speed When Passing Baseline |
|---|---|---|---|
| New (1 2/32") | Baseline stopping distance | 216 ft. | 0mph |
| Worn (4/32") | Requires 45 additional feet to stop over baseline | 261 ft. | 29mph |
| Barely legal (2/32") | Requires 71 additional feet further to stop | 287 ft. | 36mph |
Based on real-world results, Tire Rack recommends replacing tires at 4/32" tread depth if you regularly drive in rain or wet conditions.
While, these results are obviously specific to the vehicles tested, the important takeaway here is the trend: the lower tread depth is, the further it will take to stop in the wet, and the results are not subtle.
Replacing tires before they reach the bare-minimum legal depth may not seem economical, but it is far less costly than dealing with an accident.
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