CDL Knowledge Test Study Guide
Complete preparation for the CDL General Knowledge, Air Brakes, and Combination Vehicles knowledge tests — with passing score requirements and proven strategies.
1. What is a CDL?
A Commercial Driver's License (CDL) is required to operate large commercial motor vehicles (CMVs) in the United States, including semi-trucks, tractor-trailers, buses, and specialized transport vehicles. CDL requirements are federally regulated by the Federal Motor Carrier Safety Administration (FMCSA) and administered by each state's DMV.
To obtain a CDL, you must pass a series of knowledge tests at your state DMV, then demonstrate skills in a pre-trip inspection, basic vehicle controls, and an on-road driving test.
2. License Classes & Endorsements
CDL Classes
Common Endorsements
| Code | Endorsement | Vehicles |
|---|---|---|
| H | Hazardous Materials | Vehicles transporting hazmat — requires TSA background check |
| N | Tank Vehicles | Tanker trucks carrying liquids or gases |
| P | Passenger Transport | Buses carrying 16+ passengers |
| S | School Bus | Dedicated school bus endorsement |
| T | Double/Triple Trailers | Pulling double or triple trailers |
| X | Combination (Tank + HazMat) | Tanker with hazardous materials |
3. Knowledge Test Structure
| Section | Questions | Pass Score | Required For |
|---|---|---|---|
| General Knowledge | 50 | 40 (80%) | All CDL classes |
| Air Brakes | 25 | 20 (80%) | Class A, B (unless restriction) |
| Combination Vehicles | 20 | 16 (80%) | Class A only |
| Hazardous Materials (H) | 30 | 24 (80%) | Hazmat endorsement |
| Tank Vehicles (N) | 20 | 16 (80%) | Tank endorsement |
| Passenger Transport (P) | 20 | 16 (80%) | Passenger endorsement |
| School Bus (S) | 20 | 16 (80%) | School bus endorsement |
| Doubles/Triples (T) | 20 | 16 (80%) | D/T endorsement |
4. General Knowledge Section
The General Knowledge test covers the fundamentals every CDL driver must know. This is the longest section at 50 questions and covers the widest range of topics.
Key Topic Areas
Critical Numbers to Memorize
5. Air Brakes Section
The Air Brakes test is required for Class A and Class B licenses unless you accept an air brake restriction on your CDL. Passing removes the restriction and allows you to operate vehicles with air brakes — which is virtually all large commercial vehicles.
Air Brake System Components
Air Brake Inspection Tests (Pre-Trip)
Key Air Brake Numbers
6. Combination Vehicles Section
The Combination Vehicles test covers the unique challenges of operating tractor-trailer combinations. Required for Class A CDL applicants.
Coupling Procedure (Class A)
Combination Vehicle Braking
Combination vehicles with air brakes have additional considerations including trailer brake lag time and the risk of trailer jackknife. Key concepts tested:
7. Pre-Trip Inspection
The 7-step pre-trip inspection is tested in General Knowledge and forms the basis of your skills test inspection segment. Know the exact order and what to check at each step.
8. Hazmat & Other Endorsements
If you plan to haul hazardous materials (HazMat), you must pass an additional 30-question test and pass a Transportation Security Administration (TSA) background check. HazMat drivers earn significantly higher pay.
HazMat Placard Requirements
Placards must be displayed when transporting 1,001 lbs or more of any single hazard class, or any quantity of certain extremely dangerous materials (select agents like poison gases, explosives Class 1.1–1.3).
DOT Hazard Classes to Know
9. Study Strategies
Most CDL failures on the knowledge test occur because candidates memorize general concepts but miss specific numerical values that are directly tested.
High-Priority Numbers to Memorize
Study Timeline
10. Test Day Tips
How FullPracticeTests Helps You Pass CDL
Our CDL practice tests are built to match the exact format and content of your state DMV knowledge test, with full-length exams for each section and detailed explanations for every question.