For the last year or so, Michelin has been working to "start a conversation" about tire longevity—specifically, stressing that owners should be able to feel confident that their tires are safe and effective all the way down to the conventional wear limit of 2/32-inch tread depth, even in wet-weather braking. (The tire maker's research also suggested, perhaps predictably, that certain premium tires such as those Michelin make fare better in the wet than even some new tires.) The benefits of longer tire use, according to the company, are clear: fewer resources consumed and less money spent, which sounds like a no-brainer.
The AAA, however, just threw down a pointed reaction: Not so fast. Last week, the automotive safety advocacy group released its own study on the safety of worn tires that suggested no tires perform particularly well when worn down. According to its research, which included a sampling of tires at several price points, including the premium tires that Michelin sells, tires worn down to just 4/32 of an inch of tread depth can increase stopping distances by 43 percent—or an additional 87 feet, on average—compared to brand-new shoes. Not only that, but worn tires appear to result in a 33 percent reduction in handling ability for passenger cars and a 28 percent drop for light trucks. Another take-home counter to Michelin's claims: While premium tires did provide better braking performance, at just 10.2 feet less, it wasn’t statistically significant.
The AAA testing involved two vehicles: a Toyota Camry sedan and a Ford F-150 pickup truck, each tested on four sets of representative all-season tires on both a wet skid pad and a wet straight-line braking track at speeds of 60 miles per hour. (The tests were conducted, ironically enough, at Michelin’s Laurens Proving Grounds in Mountville, S.C.) Tires were tested in both new condition, and after being artificially buffed down to 4/32-inch of tread depth.
Michelin’s own tread-depth testing at the facility wasn’t quite as thorough. The research it presented publicly was actually a demonstration—not a rigorous test—conducted just on comparable premium tires, new and worn, at speeds of 45 mph. (The company has tested economy tires itself, but not published those results yet in this context.) Within its premium tire tests, it found that that “some worn tires deliver wet-braking distances that are about the same or better than other new tires.” In fact, its report stated there was a 78-foot difference in stopping distance—more than five car lengths—between the best and worst worn tire in the test.
So where does that leave consumers? Given that we don’t know the brands Michelin tested, or the details of its testing procedure—which AAA lays out in great detail, in contrast—we can’t definitively state what might account for the discrepancies in results, or even the interpretations of those results, though it’s a good bet that the variance in speeds has something to do with the former. Throw in the fact that Michelin is framing this argument as merely “starting a conversation” about how long consumers should use worn tires—a somewhat strange conversation at that, given both the safety issues at stake and that the company's business is selling tires—and the tire maker's position becomes quite a bit shakier.
When approached about the AAA study last week, Michelin argued that the study matched—in broad strokes, at least—its own argument. “Michelin has begun a conversation about worn tire performance because we believe that consumers should have information about what they can expect from their tires throughout their legal life,” the company said in a statement provided to The Drive. “AAA, like Michelin, is committed to safety, and we are pleased that AAA has added its voice to this topic. AAA’s data supports Michelin’s position that not all tires are created equal. The AAA research also supports our position that as tires wear, their wet stopping ability decreases."
"We also agree that tire degradation varies significantly among brands and even within brands," the statement went on. "In fact, some worn tires perform better than some new tires. Wear bars are legally required for all passenger tires sold in the United States. Michelin strongly believes that all tires should be designed to perform to current wear bars, thus resolving safety, financial and environmental concerns.”
We pitched the same query to AAA, which stood its ground. “AAA supports advancement in tire technology that improves safety, but AAA’s independent testing showed that all tested tires exhibited significantly diminished performance at 4/32-inch tread depth. AAA’s comprehensive wet braking testing was performed at 60 mph since this is more typical of the highway speeds motorists may encounter on their daily commute. At these higher speeds, hydroplaning is far more likely and stopping distances are dramatically longer. Demonstrations at lower speeds are interesting, but they show less variation between new and worn tires because the tire’s ability to move water is not as stressed as it is at higher speeds. AAA’s testing is not designed to evaluate a single brand, therefore we cannot comment on one.”
AAA’s testing also matches a 2014 report from Consumer Reports, which suggests wet-weather braking performance at 4/32-inch tread depth is significantly diminished—though it only tested two tires and didn’t mention the brands or price points.
Ultimately, the best course of action would probably be to err on the side of AAA’s very thorough report, given what’s at stake. Yes, your tires aren’t legally required to be replaced until they reach 2/32-inch tread depth, but there will be diminished braking performance as you get closer and closer to that. If you have premium tires, they might buy you a few feet, but it may not be worth the risk to push your tires that hard in the first place. If your tires do have a few revolutions under their belts, AAA threw in some precautions to take in the wet: avoid the use of cruise control, to better respond quickly if the car loses traction; reduce speed, and avoid hard braking and sharp turns; increase following distance to allow for greater stopping time; and if the vehicle begins to hydroplane, ease off the accelerator and steer in the direction the vehicle should go until traction is regained, but do not brake forcefully, as this can cause the vehicle to skid. The last item is one of the most important, and also one of the most counterintuitive—since slamming on the brakes is the go-to move for many drivers in an emergency. But just as is the case in snow, staying in it and steering your way out of trouble can save your butt—no matter what tread depth you’ve reached.
Our own advice: Keep your wits about you while on the road, and maintain your vehicle while off. As for the general idea of the benefits of keeping your tires on as long as possible—saving money and the environment are important, but that extra sixteenth of an inch of tire wear isn’t the battlefield we wish to die on to achieve it.