About the CDL general knowledge test
Every commercial driver's license starts with a commercial learner's permit (CLP), and every CLP starts with the general knowledge test. It's required for Class A, B and C, and it covers what every commercial driver needs regardless of vehicle: inspections, basic control, managing speed and space, seeing hazards, emergencies, cargo and fitness to drive.
States publish their own CDL manuals, but almost all are adapted from the same model manual, so the general knowledge content is consistent nationwide. Federal regulations set the floor for the test itself: at least 30 questions, with at least 80% needed to pass. Many states use a longer general knowledge test, so check your manual. Our exam mode uses the federal minimum — 30 questions at 80% — as a baseline.
Which knowledge tests you'll need
| Test | Who takes it |
|---|---|
| General knowledge | All CLP applicants |
| Air brakes | Anyone who will drive a vehicle with air brakes (otherwise an air brake restriction applies) |
| Combination vehicles | Class A applicants |
| Passenger / School bus | P and S endorsements |
| Hazardous materials | H endorsement (also requires a security threat assessment) |
| Tank vehicles | N endorsement |
| Doubles/Triples | T endorsement |
This page covers general knowledge only. Endorsement tests have their own manual chapters.
The topics, and what trips people up
Vehicle inspection
Expect specifics. Steering-axle tires need at least 4/32-inch tread and other tires 2/32. More than about 10 degrees of steering wheel play — roughly 2 inches on a 20-inch wheel — is a defect. Dual tires must not touch. Recapped tires are not allowed on a bus's front wheels. Know what you check in a pre-trip inspection and why: every item is about a failure that can kill someone.
Seeing and space management
Commercial drivers are taught to look 12 to 15 seconds ahead — about a quarter mile at highway speed — and to check mirrors constantly. Following distance is time-based and scales with vehicle length: below 40 mph, at least one second for each 10 feet of vehicle; above 40 mph, add a second. Don't trust posted overhead clearances; repaving and snow can reduce them, and an empty trailer rides higher than a loaded one.
Speed and stopping
Stopping distance is perception distance plus reaction distance plus braking distance. Braking distance grows with the square of speed, so doubling speed roughly quadruples it. Empty trucks can take longer to stop than loaded ones because they have less traction. On long downgrades, pick the low gear before starting down and use braking to control speed without overheating the brakes.
Emergencies
Steering around a hazard is often quicker than stopping. If you have to leave the road, avoid hard braking until you've slowed, and keep one set of wheels on the pavement if you can. In a skid, release the brakes to regain traction and countersteer. If brakes fail on a grade, use an escape ramp.
Warning devices
Put out warning triangles within 10 minutes of stopping. On a two-lane road: within 10 feet of the front or rear corner on the traffic side, about 100 feet behind and about 100 feet ahead. On a one-way or divided highway: 10, 100 and 200 feet behind, toward oncoming traffic. Behind a hill or curve, move the rear triangle back up to 500 feet.
Cargo
The driver is responsible for cargo even if someone else loaded it. Check securement within the first 50 miles, then every 3 hours or 150 miles, and after every break. The rule of thumb for tiedowns is one per 10 feet of cargo and never fewer than two. High centers of gravity make rollovers more likely.
Alcohol, drugs and fatigue
The commercial BAC limit is 0.04% — half the usual adult limit. Any detectable alcohol below that means 24 hours out of service. Refusing a test or driving a CMV at 0.04% or more leads to disqualification. Only sleep fixes fatigue.
Before your skills test
Since February 7, 2022, federal Entry-Level Driver Training (ELDT) rules require most first-time Class A and B applicants, and drivers adding the passenger, school bus or hazmat endorsements, to complete training with a provider listed on the FMCSA Training Provider Registry before taking the skills test (or the hazmat knowledge test). You'll also need a current DOT medical certificate. After getting your CLP, federal rules require you to hold it for at least 14 days before taking the skills test.
How to use this practice test
Take the free 15 questions, read every explanation, and use "Retry the ones I missed." Then read the matching chapters in your state's CDL manual. When you're consistently above 90%, run exam mode a few times. It's also worth a pass through the regular permit practice test and road signs quiz; general traffic law still applies to you.
Figures on this page follow the model CDL manual and federal regulations (49 CFR Part 383). Your state's manual is the final authority for its test.