CDL Exam Study Guide
Prepare for your Commercial Driver's License knowledge test with free practice tests and study guides covering general knowledge, air brakes, and combination vehicles.
- 3 Topics
- Free, online
- Plain-English explanations
Curriculum Structure
3 modules
Module 1 General Knowledge
FreeStudy general knowledge for your CDL exam. Covers vehicle inspection, basic vehicle control, CLP rules, medical certification, and disqualifying offenses.
Start with Module 1Module 2 Air Brakes
FreeStudy air brakes for your CDL exam. Covers air brake system components, spring brakes, the brake check procedure, and the L and Z restriction codes.
Module 3 Combination Vehicles
FreeStudy combination vehicles for your CDL exam. Covers the fifth wheel, coupling and uncoupling procedures, and the doubles/triples (T) endorsement.
The Full Guide
The Commercial Driver's License (CDL) knowledge test is required before you can obtain a Class A, Class B, or Class C CDL. It tests your understanding of safe vehicle operation, air brake systems, combination vehicle handling, and the federal and state rules that govern commercial driving. The general knowledge test is required for every CDL applicant, and additional knowledge tests apply if you are pursuing specific endorsements such as passenger (P), school bus (S), tank vehicle (N), or hazardous materials (H).
Class A covers any combination of vehicles with a gross combination weight rating (GCWR) of 26,001 pounds or more, provided the vehicle being towed has a gross vehicle weight rating (GVWR) of more than 10,000 pounds. Class B covers a single vehicle with a GVWR of 26,001 pounds or more, or that vehicle towing a unit of 10,000 pounds or less. Class C covers vehicles that do not meet the Class A or B definition but are designed to transport 16 or more passengers (including the driver) or that carry hazardous materials requiring placards.
Before taking the CDL skills test, most first-time applicants must complete Entry-Level Driver Training (ELDT) through a provider listed on the FMCSA Training Provider Registry, hold a Commercial Learner's Permit (CLP) for at least 14 days, and pass the required knowledge test or tests. ExamsLib provides free practice questions and study guides covering the subject areas most commonly tested on the CDL general knowledge test, air brakes test, and combination vehicles test, with plain-English explanations to help you understand the reasoning behind each answer.
ExamsLib is not affiliated with the Federal Motor Carrier Safety Administration (FMCSA), TSA, or any state driver licensing agency. Practice questions are original study material, not official exam questions. Requirements, question counts, and fees vary by state; always confirm current rules with your state's driver licensing agency before your exam.
This guide is for study purposes only. Always confirm current exam content and requirements with the official licensing authority in your state.
Frequently asked questions
What is the CDL general knowledge test?
The general knowledge test is the core knowledge test required for every CDL applicant, regardless of class. It covers safe driving practices, vehicle inspection, basic vehicle control, and the rules that apply to all commercial motor vehicles. Additional knowledge tests, such as air brakes or combination vehicles, are required depending on the vehicle you plan to operate and any endorsements you are pursuing.
Do I need Entry-Level Driver Training (ELDT) before I can get a CDL?
Yes, for most applicants. Since February 7, 2022, first-time Class A or Class B CDL applicants, drivers upgrading from Class B to Class A, and first-time applicants for the passenger (P), school bus (S), or hazardous materials (H) endorsements must complete training from a provider listed on the FMCSA Training Provider Registry before taking the relevant skills or knowledge test. Drivers who already held a valid CDL or the relevant endorsement before that date are generally exempt.
How long do I need to hold a Commercial Learner's Permit (CLP) before taking the skills test?
Federal rules require a minimum 14-day holding period from the date your CLP is issued before you are eligible to take the CDL skills test. This is a federal minimum, so no state can require less, though your state's process may take longer depending on scheduling. Separately, a CLP is valid for no more than one year from its initial issuance date, though some states issue it for a shorter term, such as 180 days, within that one-year ceiling.
What CDL endorsements are available?
Federal rules define six endorsement codes: H (hazardous materials, which also requires a TSA background check and a minimum age of 21), N (tank vehicle), P (passenger), S (school bus, which builds on the P endorsement), T (doubles/triples, knowledge test only), and X (combined tank vehicle and hazmat). Each endorsement requires passing its own knowledge test, and H, P, and S also require ELDT for first-time applicants.
What score do I need to pass the CDL knowledge test?
Federal rules set an 80 percent minimum passing score for the general knowledge test and every endorsement knowledge test. The exact number of questions on the test is not fixed federally and varies by state. Most states use around 50 questions, but Illinois uses 30 and Pennsylvania uses 70 for Class A and 50 for Class B or C. Always confirm the current question count and format with your state's driver licensing agency.
What is the difference between the L and Z restriction codes on a CDL?
Both restrictions relate to air brakes, and they are frequently confused. The L restriction is the broader one: it means the driver cannot operate any commercial motor vehicle equipped with full air brakes. The Z restriction is narrower: it means the driver cannot operate a vehicle with full air brakes but can operate one with air-over-hydraulic brakes. Either restriction is added automatically if you do not pass the air brakes portion of the knowledge test or you test in a vehicle without full air brakes.
