Tire Inspection: Tread, Inflation, and Out-of-Service Conditions
Tires are the single critical contact point between the vehicle and the road surface, and their condition directly determines traction, steering response, braking effectiveness, and fuel efficiency. The pre-trip tire inspection must cover every tire on the vehicle — steer axles, drive axles, and trailer axles — and must be thorough enough to identify conditions that would result in an out-of-service order during a roadside inspection.
Tread depth minimums under FMCSA rules are axle-position-specific. Steer axle tires (the front tires that also control steering direction) must have a minimum of 4/32-inch tread depth. All other tires (drive axles and trailer axles) must have a minimum of 2/32-inch tread depth. The driver uses a tread depth gauge to measure the depth in the major grooves of the tread — the measurement must be taken in the groove, not on the tread bar. A quarter coin inserted into the tread can serve as an informal indicator: if Washington's head is fully visible, the tread depth is approximately 2/32-inch or less.
Tire inflation is assessed visually (a properly inflated tire does not show significant sidewall bulge under load) and with an air pressure gauge on routine checks. Steer axle tires must be inflated to the load rating requirements — carrying a 12,000-pound steer axle load on a tire rated for 8,000 pounds is dangerous regardless of visual appearance. Overloaded or under-inflated tires generate excessive heat from flexing, leading to tread separation, blowout, and potential loss of vehicle control. The driver should verify that dual tire assemblies (drive axle and trailer axle duals) have matching inflation — mismatched pressure within a dual assembly causes the higher-pressure tire to carry more load, accelerating wear on that tire.
Out-of-service tire conditions include: any evidence of ply or cord exposure, cuts or snags deep enough to expose cord, bulges or knots on the sidewall or tread (indicating internal structure failure), flat or nearly flat tires (less than 50% of specified pressure), re-grooved tires on the steer axle, and any tire marked 'not for highway use' or showing similar marking. Recapped (retreaded) tires are prohibited on the steer axle but are permissible on drive and trailer axles when properly conditioned and installed.