Varying standards for minimum tread depth is something that continues to be confusing for those new to the industry. According to the Federal Motor Carrier Safety Administration (FMCSA), the minimum tread depth for truck tires is 4/32nds of an inch on the front axle and 2/32nds of an inch for all other wheel positions. Law enforcement officials often ticket drivers when the tread depths on their vehicles fall below those limits.
The Commercial Vehicle Safety Alliance (CVSA), however, states that minimum tread depths should be 2/32nds on the front axle and 1/32nd everywhere else. Authorities in the United States, Canada, Mexico and some U.S. territories use these limits to determine whether a vehicle can remain in service. If one point on one major groove on one tire is below the CVSA minimum, the vehicle can be shut down until the tire is replaced.
In addition, the issue of “one point on one major groove” is still debated in some circles because of the myth that the measurement must be found in two adjacent grooves. But the FMCSA regulation is pretty straightforward: “A vehicle does not pass an inspection if it has one of the following defects or deficiencies. …Any tire on any steering axle of a power unit with less than 4/32 inch tread when measured at any point on a major tread groove. …All tires other than those found on the steering axle of a power unit with less than 2/32 inch tread when measured at any point on a major tread groove.”
So you don't have to take my word for it: FMCSA and CVSA are both quite clear on the minimum depth of the measurements, the location of the measurements, and the number of measurements allowed below the minimum. “Major” grooves can best be described as grooves that contain a treadwear indicator or bar. While the decoupling grooves on the outside shoulders of some steer and trailer tires are not governed by minimum tread depth requirements, it's important to note that severe wear in this area can lead to belt-edge exposure, which will ultimately destroy a casing.
On the subject of retreading, in some applications it may be necessary to pull new drive tires before the tread depth reaches the 2/32nds of an inch mark. A condition called stone drilling occurs when small stones are trapped between tread blocks and the weight of the vehicle forces them into the rubber between the bottom of the groove and the top belt of the casing. If enough stones drill into the steel belts, the resulting corrosion may lead directly to the scrap pile. A retreader that recommends pulling drive tires at 4/32nds is trying to save your casings, not cheat you out of miles.
The fact that shallow tread depths create the least amount of rolling resistance, combined with the high price of diesel fuel, may tempt some fleets to push their tires right to the federal limits. Before doing that, however, keep in mind that FMCSA and CVSA both agree that any tire with one point in any major groove below the minimum is in violation, which can be expensive. While pushing tires to the limits may make sense to you, it may also lead to an increased number of casings in the scrap pile.