An interstate truck's front tire has 3/32 inch of tread in a major groove. Does it meet the federal minimum for that position?
Correct answer: No. A front tire needs at least 4/32 inch.
For a truck, truck tractor or bus subject to this rule, front-wheel tires need at least 4/32 inch in a major tread groove, measured away from tie bars, humps and fillets. A 3/32-inch reading is below that minimum.
Source: 49 CFR 393.75(b); eCFR title 49 displayed current through September 11, 2026; reviewed September 15, 2026. ↗
For a truck subject to federal tire rules, what minimum major-groove tread depth applies to a tire that is not on a front wheel?
Correct answer: 2/32 inch.
The federal minimum is 2/32 inch for these other tires. The separate front-wheel requirement is 4/32 inch. Measure in a major groove, away from raised tie bars, humps and fillets.
Source: 49 CFR 393.75(c); eCFR title 49 displayed current through September 11, 2026; reviewed September 15, 2026. ↗
A bus subject to federal equipment rules has a retreaded front tire with deep, undamaged tread. Is the tread depth enough to make that installation acceptable?
Correct answer: No. Retreaded tires are prohibited on the front wheels of the bus.
Federal rules prohibit regrooved, recapped and retreaded tires on the front wheels of a bus. Adequate tread depth does not remove that position-specific restriction; this is not a ban on every rear tire.
Source: 49 CFR 393.75(d); eCFR title 49 displayed current through September 11, 2026; reviewed September 15, 2026. ↗
On a commercial vehicle subject to federal tire rules, a sidewall cut exposes the tire's reinforcing material. What should the driver do?
Correct answer: Have the unsafe tire corrected before operating the vehicle.
Exposed body-ply or belt material is a prohibited tire condition, whether exposed through the tread or sidewall. Tread depth and the presence of a neighboring tire do not make that tire safe.
Source: 49 CFR 393.75(a)(1), (a)(4); eCFR title 49 displayed current through September 11, 2026; reviewed September 15, 2026. ↗
During a walk-around, you notice new rust streaks extending from wheel nuts. What do they suggest?
Correct answer: The nuts may be loose and their tightness needs checking.
Rust trails can indicate movement at a wheel nut. Check the assembly rather than dismissing the streaks as cosmetic. The handbook also calls for a tightness recheck after a wheel has recently been changed.
Source: Missouri Commercial Driver License Manual, revised August 2025, Section 2 (July 2017); 2.1.3 Wheel and Rim Problems; physical PDF page 18 (printed 2-2); reviewed September 15, 2026. ↗
Two tires in a dual assembly are touching each other. How should you treat this finding?
Correct answer: As a tire defect to resolve before the vehicle is driven.
Dual tires should not rub against each other or vehicle components. Matching size and apparently good tread do not cancel the need to investigate contact and correct the unsafe condition.
Source: Missouri Commercial Driver License Manual, revised August 2025, Section 2 (July 2017); 2.1.3 Tire Problems; physical PDF page 17 (printed 2-1); reviewed September 15, 2026. ↗
You find grease on a truck's brake lining during inspection. What is the appropriate response?
Correct answer: Have the brake defect corrected before starting the trip.
Oil, grease or brake fluid on friction material is a brake defect. A sound drum does not make contaminated lining acceptable, and the parking brake is not a substitute for a safe service-brake system.
Source: Missouri Commercial Driver License Manual, revised August 2025, Section 2 (July 2017); 2.1.3 Bad Brake Drums or Shoes; physical PDF page 18 (printed 2-2); reviewed September 15, 2026. ↗
Why is a cracked spring hanger more serious than a comfort problem?
Correct answer: It can allow the axle to move out of its proper position.
The suspension supports the vehicle and load while keeping axles correctly positioned. Damaged hangers or related mounting parts can threaten control and require attention, even before several spring leaves have failed.
Source: Missouri Commercial Driver License Manual, revised August 2025, Section 2 (July 2017); 2.1.3 Suspension System Defects; physical PDF page 18 (printed 2-2); reviewed September 15, 2026. ↗
A driver notices an exhaust leak beneath the sleeper area. Why must it be addressed?
Correct answer: Poisonous exhaust can enter the cab or sleeper.
A damaged exhaust system can let poisonous fumes into occupied spaces. Check leaking or unsecured exhaust components and have the defect corrected; opening a window is not a repair.
Source: Missouri Commercial Driver License Manual, revised August 2025, Section 2 (July 2017); 2.1.3 Exhaust System Defects; physical PDF page 19 (printed 2-3); reviewed September 15, 2026. ↗
Before climbing into a parked truck, which observation deserves investigation?
Correct answer: A fresh fluid puddle under the engine area.
The approach inspection looks for clues such as fresh leaks, damage or an unusual lean, as well as people and obstacles around the vehicle. Investigate a fresh puddle instead of discovering the problem after departure.
Source: Missouri Commercial Driver License Manual, revised August 2025, Section 2 (July 2017); 2.1.5 Approaching the Vehicle; physical PDF page 19 (printed 2-3); reviewed September 15, 2026. ↗
A driver checks the same vehicle daily. What approach best reduces overlooked inspection items?
Correct answer: Use a consistent inspection routine on each trip.
A repeatable method helps prevent omissions. Familiarity with a vehicle or a trouble-free previous trip does not show that its current condition is safe.
Source: Missouri Commercial Driver License Manual, revised August 2025, Section 2 (July 2017); 2.1.5 Method of Inspection; physical PDF page 19 (printed 2-3); reviewed September 15, 2026. ↗
A loose tool bag is resting where it could slide under the pedals. When should you secure it?
Correct answer: Before moving, so it cannot obstruct the controls.
Loose cab objects can interfere with driving controls or strike occupants in a crash. Secure them before driving rather than relying on their current position.
Source: Missouri Commercial Driver License Manual, revised August 2025, Section 2 (July 2017); 2.1.5 Get In Vehicle; physical PDF page 23 (printed 2-7); reviewed September 15, 2026. ↗
