Vehicles used:
2006 BMW 325i

Is Washington's quarter a safer measure of tread depth for tires driven in dry road conditions?

Following Tire Rack's report on the negative influence insufficient remaining tread depth has on a tire's wet braking distances, "What Honest Abe Doesn't Tell You About Minimum Tread Depths", some drivers felt our report was incomplete because it didn't examine a shallow treaded tire's positive influence on performance in dry conditions. So we felt it was appropriate to conduct a series of dry braking tests to round out our evaluation.

Testing the Premise

We used one of Tire Rack's 2006 BMW 325i test cars equipped with 4-wheel vented disc brakes and an Antilock Braking System (ABS). The BMW was fitted with same tires we had run previously in the wet braking tests, the 205/55R16 91H-sized Michelin Energy MXV4 Plus previously used as Original Equipment (O.E.) on some BMW 3-Series cars.

One set of test tires remained at the full, 10/32" of original tread depth, while additional sets had been shaved to nearly 4/32" (easily measured using a Washington quarter) and 2/32" of remaining tread depth (traditionally measured using a Lincoln penny and almost legally worn out in most states).

The shaved tires were baked in an industrial oven for several weeks at temperatures that would help replicate the rubber aging experience of tires resulting from several years of driving and wear. The final step of initial tire preparation was to break all of the tires in by driving them approximately 100 highway miles. The tires were then used for two wet stopping evaluations before being used in this dry stopping test.

We conducted the dry braking evaluation on Tire Rack test track and recorded the tires' panic stop performance using our standard 50-0 mph format. This format ensured that all tires began braking at equivalent speeds and experienced the same conditions. We recorded the tires' braking performances with our Vericom VC2000 and DriftBox performance analyzers. The Vericom was used to record the limits of performance and the DriftBox was used to confirm accurate test speeds and chart our results.

Braking Results

Since we normally test new tires with full tread depths, the braking forces developed by the new 10/32" deep Energy MXV4 Plus tires felt reminiscent of the full tread depth Grand Touring tires we'd experienced previously. The car pitched forward as the brakes were applied, quickly took a set and slowed the car with authority. After repeated runs we learned that the average stopping distance from 50 mph for the vehicle equipped with new tires was 89.3-feet.

However following the same procedures with the worn tires revealed a difference even before the panic stop braking commenced. Preparation for our braking test requires accelerating the vehicle from 40 mph to 50 mph while driving through a large, sweeping 325-foot diameter turn that moderately challenges the tires' steering response and handling. Because brake testing requires consistent repetition of driving technique, our tester was able to notice an improvement in steering responsiveness and cornering stability with the 4/32" deep tires. While this wasn't unexpected, the real question was how much of this enhanced handling feel would translate into shorter stopping distances. It didn't take long to get the answer. After repeated runs on the 4/32" deep tires, we learned that the average stopping distance from 50 mph was 87.8-feet, just 1.5-feet shorter than with the full tread depth tires.

When evaluations with the 2/32" deep tires commenced, our tester was looking to see if another incremental improvement in steering response and cornering stability would be felt. The 2/32" deep tires provided a very subtle handling improvement, yet after repeated runs, we learned that their average stopping distance from 50 mph was 87.8-feet, the same distance as the 4/32" deep tires.

Looking at the graphs recorded by the DriftBox confirmed slight differences in dry conditions. The graphs we saw showed that the panic stopping capabilities of the new tires were roughly equivalent to worn and nearly worn out tires in dry conditions.

Rewriting History

Though dry braking isn't significantly affected by tire tread depths, if rain and wet roads are a concern, Tire Rack suggests that drivers consider replacing their tires when they reach approximately 4/32" of remaining tread depth. Since water can't be compressed, tires need enough tread depth to allow rain to escape through their grooves. If the water can't escape fast enough, the vehicle's tires will be forced to hydroplane (float) on top of the water, losing traction and increasing stopping distances. Fortunately, measuring 4/32" of remaining tread depth with a U.S. coin would simply require replacing the outdated Penny Test with an up-to-date Quarter Test (measuring with the distance from the coin's circumference to the top of Washington's head).

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 any of these reasons exceed the emotional and monetary costs of drivers and passengers recuperating from injuries or repairing a vehicle after an accident.

Some tire manufacturers will provide a consumer concession on a one-by-one basis that allows tire removal before reaching 2/32" for drivers facing seasons of the year that are accompanied by significant rain and snow. We believe the purpose of these consumer concessions needs to be formalized and be written into all tread life mileage warranties. We know these changes won't happen overnight so we're encouraging all drivers to pay more attention to their tires now.