The DOT legal tread depth minimum is 2/32 of an inch for passenger vehicle tires and for most tire positions on commercial vehicles. Front (steer) tires on commercial trucks and buses have to hold at least 4/32 of an inch. Once a tire wears to those levels, it is legally bald and has to come off the road. Most tire and safety experts recommend replacing well before you hit either number.
The 2/32-Inch Passenger Car Rule
The 2/32-inch threshold comes from federal testing that found tires lose traction rapidly once they wear below that point.1National Highway Traffic Safety Administration. Interpretation 11497AWKM Every passenger tire sold in the United States is required to have treadwear indicator bars molded into the tread grooves at that depth. They are small rubber bridges at the bottom of the grooves. When the surrounding tread wears flush with them, the tire has reached its legal limit.
NHTSA sets the manufacturing rule that puts those wear bars on the tire. Enforcement of when a worn tire must be pulled from the road belongs to the states. NHTSA publishes model inspection guidelines at 49 CFR Part 570, and most states have adopted the 2/32-inch cutoff.1National Highway Traffic Safety Administration. Interpretation 11497AWKM
Commercial Vehicle Tread Depth
Commercial motor vehicles operate under a stricter, federally enforced standard at 49 CFR 393.75. Front-axle tires on any bus, truck, or truck tractor have to maintain at least 4/32 of an inch of tread.2eCFR. 49 CFR 393.75 – Tires The front axle steers the vehicle, and a blowout or traction loss there on a loaded truck creates far more danger than the same failure on a rear axle.
All other tire positions on commercial vehicles, including drive axles and trailer axles, have to hold at least 2/32 of an inch.2eCFR. 49 CFR 393.75 – Tires The regulation also bars regrooved, recapped, or retreaded tires on the front wheels of any bus.
Measurements have to be taken in the major tread grooves, not where tie bars, humps, or fillets sit.2eCFR. 49 CFR 393.75 – Tires For commercial vehicles, inspectors check two adjacent grooves at three points spaced roughly evenly around the tire. One good spot is not enough to pass.
How to Check Your Own Tread
Three methods work for everyday drivers.
- Look at the wear bars. Find the raised rubber bridges inside the main grooves. When the tread around them is level with the top of the bar, you are at 2/32 of an inch.
- Try a penny. Put a penny into a major groove with Lincoln’s head pointing down. If you can see the top of his head, the tread is under 2/32 of an inch and the tire fails the legal standard.
- Try a quarter. Same idea with Washington’s head down. If the tread does not reach the top of his head, you are below roughly 4/32 of an inch. This is the quick check for commercial steer tires and for gauging whether passenger tires are getting close to the safety margin.
A tread depth gauge gives an exact reading in thirty-seconds of an inch. Push the base flat against the tread and read the scale. It is the most precise option and the one inspectors and shops use.
Why 2/32 Is Not the Safe Number
The legal minimum is a floor, not a performance threshold. Traction fades gradually as tread wears, and the difference between 4/32 and 2/32 is significant in wet weather. Shallow tread cannot channel water out of the contact patch, so stopping distances get longer and the hydroplaning risk climbs.
New passenger tires typically start with 10/32 to 11/32 of an inch. By 4/32, the tire has already lost most of its wet-weather capability. If you drive in rain often, replacing at 4/32 rather than waiting for the legal limit gives you a real safety margin. Drivers in dry climates can run closer to the legal number with less risk, but the tire is still performing worse than it did a few thousand miles earlier.
Age Matters as Much as Tread
Tread depth is not the whole story. Rubber degrades over time regardless of tread. A tire sitting in a garage for years with full tread can still be structurally compromised because the compounds break down from oxidation and UV exposure.
Every tire has a DOT Tire Identification Number stamped on the sidewall. The last four digits are the manufacture date: the first two are the week, the last two are the year. A tire stamped “1523” was made in the 15th week of 2023. Most vehicle and tire manufacturers recommend replacement at six to ten years old, even if the tread looks fine. Check the date code on any tire you buy, including a spare, since tires can sit in warehouses before they are sold.
What Happens If You Drive on Bald Tires
Penalties vary by state, but expect a traffic citation and fine if an officer spots bald tires during a stop. In states with periodic safety inspections, tires below the legal minimum fail the inspection, and you generally cannot renew your registration until the tires are replaced and the vehicle passes.
Commercial drivers face heavier consequences. A steer tire below 4/32, or any other tire below 2/32, can trigger an out-of-service order under the North American Standard Out-of-Service Criteria, which keeps the vehicle from moving until the tires are replaced.3Commercial Vehicle Safety Alliance. Out-of-Service Criteria
There is also civil exposure. If you are in a crash and your tires are found to be below the legal tread depth, that fact can be used against you in insurance claims and lawsuits. Arguing you did not know the tires were unsafe is hard when the wear bars are visibly flush with the tread.