CDL Written Test Guide: What to Study and How to Pass

Understanding the CDL Written Tests

The CDL written tests (also called knowledge tests) are the first testing hurdle in getting your CDL. You take them at your state DMV to earn your Commercial Learner's Permit (CLP). The tests are multiple-choice, computer-based in most states, and you need a score of 80% or higher to pass each one.

Every CDL applicant takes the General Knowledge test (50 questions). If you are pursuing Class A, you also take the Combination Vehicles test (20 questions). If your vehicle has air brakes (most commercial trucks do), you take the Air Brakes test (25 questions) — failing this one puts an air brake restriction on your CDL.

Endorsement tests are optional but valuable: Hazmat (H) is 30 questions, Tanker (N) is 20, Doubles/Triples (T) is 20, Passenger (P) is 20, and School Bus (S) is 20. You can take multiple endorsement tests on the same visit. My advice: take them all now, even if you do not plan to use them immediately. The knowledge is fresh, and having endorsements on your license makes you more marketable.

The passing score of 80% means you can miss roughly 10 questions on the General Knowledge test and still pass. That sounds generous, but the questions cover a wide range of material, so thorough preparation is essential.

General Knowledge Test Breakdown

The General Knowledge test is the broadest section, covering everything from vehicle inspection to cargo securement to driving in adverse conditions. Here are the major topic areas and their approximate weight on the test.

Vehicle inspection and maintenance (15-20% of questions): pre-trip inspection procedures, tire conditions, fluid checks, coupling device inspection, and recognizing defects that put a vehicle out of service. Know the seven-step inspection method and what critical items to check at each position around the vehicle.

Safe driving practices (20-25%): following distance, speed management, space management, night driving, mountain driving, and skid recovery. The key concept is the Smith System — aim high in steering, get the big picture, keep your eyes moving, leave yourself an out, and make sure they see you.

Cargo handling and securement (10-15%): weight distribution, securing cargo with chains/straps/blocking/bracing, covering loads, and understanding cargo weight limits. Know that cargo must be inspected within the first 50 miles and every 3 hours or 150 miles thereafter.

Federal regulations (15-20%): hours of service (HOS) rules, CDL disqualifications, alcohol and drug testing requirements, and hazardous materials basics. Memorize the current HOS limits: 11-hour driving limit, 14-hour on-duty limit, 30-minute break requirement, and 70-hour/8-day limit.

Air Brakes Test: The Section Most People Fail

The Air Brakes test has the highest failure rate of any CDL written test, and for good reason — air brake systems are complex and the questions are technical. Do not underestimate this section. If you fail it, your CDL will have an air brake restriction, which disqualifies you from driving virtually all commercial trucks.

Key concepts you must understand: how air compressors build pressure, the role of the governor (typically cuts in at 100 psi and cuts out at 125 psi), how the air dryer removes moisture, and how the safety valve releases at 150 psi to prevent tank rupture.

Brake components are heavily tested. Know the difference between S-cam brakes and wedge brakes, understand how slack adjusters work, and memorize the function of brake drums, linings, and return springs. For the air brake application: pressing the brake pedal pushes air through brake lines to brake chambers, which push push rods that turn slack adjusters that twist S-cams that spread brake shoes against the brake drum.

The most commonly tested facts: air loss rate should not exceed 3 psi per minute for single vehicles or 4 psi for combination vehicles with the engine off and brakes applied. The low air pressure warning should activate before air pressure drops below 60 psi. Spring brakes (parking brakes) automatically engage when air pressure drops to 20-45 psi. The applied pressure test (brake pedal held for one minute) should show no more than a specific drop.

Study air brake diagrams until you can trace the airflow from compressor to wheels. Understanding the system conceptually makes memorizing specific numbers much easier.

Combination Vehicles Test (Class A Only)

The Combination Vehicles test is required only for Class A applicants and covers the unique aspects of driving tractor-trailers and other combination vehicles. This is a 20-question test that focuses on coupling/uncoupling, trailer stability, and combination-specific safety concerns.

Coupling and uncoupling procedures are the biggest topic. You need to know the correct step-by-step sequence for connecting and disconnecting a tractor from a trailer. Common test questions focus on the order of operations: when to connect air lines, when to test the connection, and what happens if you couple incorrectly (the trailer drops, which can be fatal).

Trailer stability topics include understanding rollover risk (combination vehicles have a higher center of gravity), off-tracking (the trailer wheels follow a tighter path than the tractor in turns), and the crack-the-whip effect on double and triple trailers. Know that the rearmost trailer in a doubles/triples combination is the most likely to roll over.

Jackknifing is heavily tested. A jackknife occurs when the drive wheels lose traction and the trailer pushes the tractor sideways. Causes include braking too hard on slippery surfaces, driving too fast for conditions, and improper trailer loading. Recovery involves releasing the brakes and steering in the direction of the skid.

Air brake questions specific to combination vehicles cover the tractor protection valve (which prevents air loss if the trailer breaks away), the trailer air supply valve, and the proper testing procedure for combination vehicle brakes.

Proven Study Strategies That Actually Work

The most effective study approach combines reading the official CDL manual with repeated practice testing. Start by reading your state's CDL manual cover to cover — it is free from your DMV website and contains everything you need to know. Most manuals are 100 to 150 pages.

After your first read-through, take a full practice test. Do not worry about your score — the goal is to identify your weak areas. Every question you miss tells you what section to re-study. Most free practice test websites categorize questions by topic, making it easy to drill specific areas.

Flashcards work well for memorizing specific numbers (psi values, distances, time limits, weight limits). Create cards for every specific number mentioned in the manual. There are roughly 50 to 75 specific numerical values you need to memorize across all sections.

Study in 30 to 45-minute sessions with breaks between them. Research consistently shows that spaced repetition (studying a little each day over two weeks) produces better retention than marathon cramming sessions. Plan for 2 to 3 weeks of study time.

The day before your test, do a final review of your weak areas and take 2 to 3 full practice tests. If you are consistently scoring 90% or above on practice tests, you are ready. If any section is below 85%, spend more time on it.

At the DMV, read each question carefully — many wrong answers come from misreading. Watch for qualifier words like "always," "never," "except," and "most." If you are unsure, eliminate obviously wrong answers first. Flag difficult questions and come back to them after completing the easier ones.