2/32nds or 4/32nds? It’s the degree of safety that needs to be discussed

March 6, 2009

Driving on a tire with 2/32nds-inch tread depth is safe. Driving on a tire with 4/32nds-inch tread depth is safe.

That seems to be the consensus in our industry. The federal government only requires that tread wear bars be placed at the 2/32nds-inch wear mark. It’s up to the states to set a minimum tread depth standard, and not one of the 50 states has a standard higher than 2/32nds-inch.

In Europe, the issue is more concrete. The minimum tread depth standard is 1.6 millimeters — roughly 2/32nds-inch.

So, what’s all the fuss about tread depth? Why do some people want to replace tires at the 4/32nds-inch wear mark? It all centers on safety, according to The Tire Rack and Continental AG.

Tire Rack started the tire rolling last fall when it ran a video on its Web site demonstrating the effects of tread depth on wet braking. (To view the video, visit tirerack.com/treaddepthtest.) As a result of its own testing, Tire Rack recommends changing tires at 4/32nds-inch.

“While there are obvious trade-offs associated with replacing tires before they are fully worn out (such as an increase in driving cost per mile and the reality that more tires would be discarded annually), we don’t believe these reasons exceed the costs to repair a vehicle (or its occupants) after an accident,” wrote the company in its Inside Track catalog.

The implication is obvious, although Tire Rack stops short of calling for government intervention.

Continental recently became the second tire manufacturer to definitively and publicly address the issue, after Michelin North America Inc. At a press conference in Korbach, Germany, Dr. Burkhard Wies, vice president of passenger and light truck tire line development worldwide, said tires should be removed at 3 mm (4/32nds-inch) for safety’s sake. He said research clearly shows that a tire has significantly greater wet traction at 4/32nds-inch than at 2/32nds-inch.

Michelin agrees with those findings. However, it does not believe anyone should mandate the removal of tires before 2/32nds-inch.

The company says there is no accident data proving that the difference in tread depth has a negative impact on safety.

Lacking this proof, other factors need to be considered, including the environmental impact and capacity constraints, says Michelin.

Wies presented research from RWTH Aachen University in Aachen, Germany, that showed how accident probability decreases as a tire’s road friction value increases. Common sense tells you there will be a negative impact on safety between 4/32nds-inch and 2/32nds-inch tread depth, he added.

My feeling is that Wies is correct about an expected increase in accidents and fatalities with the drop in tread depth.

However, it’s not about safety. It’s about the degree of safety. And that’s where the debate becomes less clear-cut.

Why not take the tires out of commission at 6/32nds-inch? Certainly the tires will stop sooner. Wouldn’t that save more lives? How about 8/32nds-inch? Same thing.

But there has to be a balance between consumer wants and needs, and — I’ll say it — that includes price.

Consumers are not going to want to take the time or spend the money to change tires as often as they change their oil. And what about people with low incomes? Used tires with 4/32nds-inch tread depth or less are an option to them.

“If you want safety, you have to pay for it — no doubt,” said Wies.

With the cost of gas skyrocketing, people are holding off on replacing their tires until they have to. 

Regardless of how well you want a tire to perform, there is no way it can be designed to prevent accidents under all conditions. There are too many variables to consider. So safety versus economy is a legitimate debate at any tread depth.

As for me, I would like to see a little more credible data before deciding. Either way, I definitely like the idea of the issue being discussed internally, without government involvement.

Kudos to Tire Rack, Michelin and Continental for getting the dialogue started, and keeping the discussion in house.    ■

About the Author

Bob Ulrich

Bob Ulrich was named Modern Tire Dealer editor in August 2000 and retired in January 2020. He joined the magazine in 1985 as assistant editor, and had been responsible for gathering statistical information for MTD's "Facts Issue" since 1993. He won numerous awards for editorial and feature writing, including five gold medals from the International Automotive Media Association. Bob earned a B.A. in English literature from Ohio Northern University and has a law degree from the University of Akron.