The Commercial Driver's License (CDL) knowledge tests are written from the Commercial Driver's License Manual — a federal document standardized by the Federal Motor Carrier Safety Administration (FMCSA) but published and distributed by each state's Department of Motor Vehicles or equivalent agency. The CDL manual is the single most important study resource for all CDL knowledge tests. Every question on every CDL knowledge test — General Knowledge, Air Brakes, Hazardous Materials, Combination Vehicles, and all other endorsements — comes from the CDL manual.
To obtain the CDL manual for your state, visit your state's DMV or motor vehicle agency website and download the commercial driver's license handbook or manual — it is available free as a PDF. While the core content of the CDL manual is federally standardized (the regulations governing commercial vehicle operation are federal), individual state editions may include state-specific laws and procedures. Download your state's edition rather than using a generic version, as questions may reflect state-specific content.
The CDL manual covers several distinct sections, each corresponding to a separate knowledge test. The General Knowledge section is the most comprehensive and covers: vehicle inspection, basic vehicle control, shifting and backing, cargo securement and weight limits, passenger and school bus operations, pre-trip inspection procedures, driving safely, hazardous conditions and emergency maneuvers, and federal regulations governing commercial vehicle operation. Additional sections cover air brakes, combination vehicles, tankers, doubles and triples, hazardous materials, passenger vehicles, and school buses — each of which is tested separately for drivers seeking those certifications or endorsements.
The CDL General Knowledge test is required for all CDL applicants and covers fundamental commercial vehicle operation knowledge. It is the foundation upon which all other CDL tests build. The General Knowledge test typically consists of 50 questions with a passing score of 80% (40 correct answers). Questions come directly from the CDL manual's general knowledge section.
Several topic areas appear repeatedly on CDL General Knowledge tests and deserve dedicated study: pre-trip inspection procedures (what you must inspect, the correct sequence, and what disqualifies a vehicle from service); cargo securement requirements (tie-down ratios, load distribution, blocking and bracing, cargo weight limits and axle weight maximums); vehicle systems knowledge (brake systems, coupling and uncoupling, fifth wheel inspection); federal hours-of-service rules (maximum driving hours, mandatory rest periods, log book requirements); and hazardous conditions response (skid recovery, hydroplaning, emergency stopping). The pre-trip inspection section is particularly detailed and heavily tested — drivers are expected to know what to inspect under the hood, around the vehicle, and inside the cab.
Federal hours-of-service (HOS) regulations are tested on the General Knowledge exam because all commercial drivers must comply with them. Key HOS rules: drivers may drive a maximum of 11 hours after 10 consecutive hours off duty (property-carrying drivers); the 14-hour rule limits the window of on-duty time including driving; the 30-minute break requirement applies after 8 hours of driving; the 60/70-hour limit applies within a 7 or 8 consecutive day period. These specific numbers are testable — memorize them from the regulations section of the CDL manual.
In addition to the General Knowledge test and Air Brakes test (if applicable), CDL holders who want to drive specialized vehicles must pass endorsement knowledge tests. Each endorsement covers a specific vehicle or cargo type and requires passing a separate multiple-choice test at the DMV.
The Hazmat endorsement is one of the most extensively tested CDL endorsements, covering federal regulations for transporting hazardous materials — flammables, explosives, radioactive materials, poisons, and other dangerous goods. Hazmat testing covers: the hazardous materials table, placarding requirements (which hazmat classes require which placards, and when placards are not required), shipping papers requirements, loading and unloading procedures, emergency response, and driver responsibilities. The Hazmat endorsement also requires a TSA security threat assessment (fingerprinting and federal background check) because hazmat transport is regulated for national security reasons. The Hazmat test typically has 30 questions with an 80% passing requirement.
The Tanker endorsement covers operating liquid and gaseous cargo tanks — principles of liquid surge (forward and rearward surge when stopping or accelerating), tank inspection, loading and unloading procedures, and emergency procedures specific to tank vehicles. The Combination Vehicles endorsement covers coupling and uncoupling procedures for tractor-trailer combinations, including fifth wheel inspection, sliding tandem axles, and doubles and triples handling characteristics. Doubles and Triples (T) endorsement covers the specific handling and inspection requirements for pulling two or more trailers simultaneously.
Passenger and School Bus endorsements require specialized knowledge about passenger safety, emergency exits, and specific handling rules for their vehicle types. The School Bus endorsement is among the most detailed — school bus drivers must understand loading and unloading zones, railroad crossing procedures for school buses (always stop at railroad crossings, regardless of signals), emergency exit routes, student management, and mirror adjustment for maximum visibility around the bus. These tests are typically 20 to 30 questions each.
The Air Brakes test is technically not an endorsement but a restriction removal — CDL holders who do not pass the Air Brakes test receive a restriction on their license prohibiting them from operating vehicles with air brakes. The Z restriction (no air brakes) significantly limits employment options, as most heavy commercial vehicles use air brake systems. Passing the Air Brakes test removes this restriction.
The Air Brakes knowledge test covers: how air brake systems work (the air pressure system, compressors, air reservoirs, service brakes, parking brakes, emergency brakes); the pre-trip inspection of air brake components (including how to check brake adjustment, air loss rate test, and low air pressure warning); safe driving with air brakes (adjusting stopping distance for air brake lag, controlled braking on downgrades, brake fade); and air brake system failures and emergency responses. Air brakes require different inspection and driving procedures than hydraulic brakes — drivers must understand the system thoroughly, not just the basics. The Air Brakes test typically has 25 questions with an 80% passing requirement.
Drivers pursuing the Combination Vehicles endorsement (tractor-trailer, doubles, triples) must also pass the Air Brakes test, as all combination vehicles use air brake systems. Study Air Brakes and Combination Vehicles chapters concurrently — the topics overlap significantly in how they relate to vehicle operation and inspection.
Passing CDL knowledge tests requires systematic study of the CDL manual and targeted practice testing. The following strategy works for first-time CDL applicants across all states.
Download the CDL manual from your state's DMV website as the first step. Read the manual in the order of the knowledge tests you need to pass: start with General Knowledge (required for all CDL classes), then Air Brakes (if applicable), then any endorsements you need. Read each section actively — take notes on numbers, specific rules, and procedures that are testable. Highlight requirements and restrictions rather than descriptive explanations.
After reading each major section of the CDL manual, take practice tests covering that section before moving on. This reinforces retention and identifies gaps while the material is fresh. Staggered practice — read, test, review, then read next section — is more effective than reading the entire manual and then practicing at the end. Review every wrong answer against the specific manual passage that explains the rule before your next practice session.
Continue practicing until you consistently score 90% or higher on practice tests for each section you need to pass. The actual CDL knowledge tests require 80%, so aiming for 90% on practice tests gives you buffer for the stress and unfamiliar phrasing of the real test. Schedule your DMV knowledge test appointment only after reaching this preparation level. Most states allow CDL knowledge tests to be taken on a walk-in or appointment basis — check your state's specific process for commercial knowledge testing.