The Short Answer
Your CDL class is set by the vehicle, not by you: Class A is a combination of vehicles rated 26,001 lb or more with a trailer over 10,000 lb, Class B is a single heavy vehicle rated 26,001 lb or more, and Class C is anything smaller that carries 16 or more passengers or hazardous materials. Everyone takes the general knowledge test. Class A adds the combination vehicles test. Air brakes are tested on top if the truck has them, and each endorsement adds its own test.
Federal rules set the same bar for all of them: at least 80% on each knowledge test, scored separately (49 CFR 383.135). They do not set how many questions each test has — your state does.
Class A, Class B and Class C: What Each Covers
The three classes are the “commercial motor vehicle groups” in 49 CFR 383.91. The words below follow the regulation:
| Class | Vehicle | For example |
|---|---|---|
| A — combination | Combination of vehicles with a gross combination weight rating (GCWR) of 26,001 lb or more, where the towed vehicle is rated over 10,000 lb | A tractor-trailer |
| B — heavy straight | A single vehicle with a GVWR of 26,001 lb or more, or one towing a vehicle rated 10,000 lb or less | A large straight truck or bus |
| C — small | Any vehicle or combination that is neither A nor B, but is designed to carry 16 or more passengers including the driver, or is used to carry hazardous materials | A smaller bus or a placarded hazmat vehicle |
A higher class covers the lower ones. A Class A driver who holds the right endorsements can drive Class B and Class C vehicles, and a Class B driver can drive Class C (383.91(c)). Going the other way means retaking the related tests.
Which Knowledge Tests Each Class Takes
| Class | Always | Adds |
|---|---|---|
| A | General knowledge | Combination vehicles; air brakes if the vehicle has them |
| B | General knowledge | Air brakes if the vehicle has them |
| C | General knowledge | The endorsement tests that match the vehicle (passenger or hazmat); air brakes if it has them |
The general knowledge test covers 20 areas of safe operation, from vehicle inspection and basic control to space management, night driving and emergencies (383.111(a)). Our CDL general knowledge guide walks through it and the numbers that decide most failures.
Most Class A candidates sit general knowledge, air brakes and combination in the same visit. Each is scored on its own, so passing one does not carry another.
The Combination Vehicles Test (Class A)
Anyone driving a combination vehicle has to know three things (383.111(c)): how to couple and uncouple a tractor and semi-trailer, the inspection steps unique to combination vehicles, and general operating practice, including air brakes. Fail this test and the State must not issue you a Class A permit or licence (383.135(a)(3)). It is the one failed test that blocks a whole class.
The Air Brakes Test, and the Restriction If You Fail
Every driver of a vehicle with air brakes needs the air brake knowledge. The regulation lists seven areas (383.111(b)): air brake system nomenclature, the dangers of a contaminated air supply, what severed or disconnected air lines mean, what low air pressure readings mean, pre-trip inspection of the system, en route and post-trip inspection (including the air-loss and low-pressure warning tests), and general operating practice such as braking technique, antilock brakes, emergency stops and the parking brake.
Failing does not stop you getting a licence. It restricts it: if you fail the air brake knowledge test, or take your skills test in a vehicle without air brakes, the State must mark your permit or licence as barred from any vehicle with air brakes (383.95(a)). That is the L restriction. “Air brakes” here means any system that works fully or partly on the air brake principle.
A skills test in a truck with air-over-hydraulic brakes adds the Z restriction, which bars vehicles with full air brakes (383.95(b)). Take the skills test in a truck that matches what you will drive.
Endorsements: The Extra Tests
An endorsement lets you drive a particular kind of vehicle and needs its own test (383.93). The letters below are the codes printed on the licence (383.153):
| Code | Endorsement | Tests |
|---|---|---|
| T | Double/triple trailers | Knowledge |
| P | Passenger | Knowledge and skills |
| S | School bus | Knowledge and skills, on top of the passenger endorsement |
| N | Tank vehicle | Knowledge |
| H | Hazardous materials | Knowledge; also needs TSA approval |
| X | Tank vehicle and hazardous materials combined | The N and H tests |
Only three endorsements are allowed on a commercial learner’s permit: passenger, school bus and tank vehicle (383.93(a)(2)). Hazmat needs TSA approval before the State can issue it (49 CFR 383, Subpart I). Entry-level driver training is required before the skills test for a first Class A or B licence, a B-to-A upgrade, or a first S, P or H endorsement (49 CFR Part 380).
Other Restrictions to Know
Where you take the skills test can limit the licence. Besides L and Z above, the codes you may see are (383.95, 383.153):
- E — no manual transmission, if you tested in an automatic.
- O — no tractor-trailer, if your Class A skills test used a pintle-hook connection instead of a fifth wheel.
- M and N — no Class A passenger vehicle, and no Class A or B passenger vehicle, if your passenger skills test used a smaller bus. This N is a restriction, not the tank endorsement.
How to Practise for Each Test
- Work out your class from the vehicle you will drive, then list the tests it needs from the table above.
- Read the matching sections of your state’s CDL manual once through. Every state builds its manual and tests from the same model, and the manual is the source of every question.
- Practise the general knowledge test first, because everyone takes it. QuizCram’s CDL practice test is written from the federal knowledge-test requirements; the first 20 questions are free with the answer, an explanation and the source on each.
- Our bank is a general knowledge bank. It includes questions on braking, doubles and triples, passenger and hazmat topics, but it is not a separate air brakes or combination vehicles test. Use your state manual for the full air brake and coupling sections.
- Aim for 90% or better twice in a row before you book. The real pass mark is 80%, and the margin absorbs the questions you have not seen.
CDL Class and Endorsement FAQ
Which CDL class do I need?
It depends on the vehicle. Class A for a combination of vehicles rated 26,001 lb or more with a trailer over 10,000 lb; Class B for a single vehicle rated 26,001 lb or more; Class C for smaller vehicles that carry 16 or more passengers or hazardous materials (49 CFR 383.91).
How many questions are on the air brakes test?
Federal rules do not fix a number. They fix the pass mark, 80%, and the seven knowledge areas. Your state decides how many questions the test has, so check your state CDL manual or licensing agency.
What happens if I fail the air brakes test?
You can still get a licence, but it carries an L restriction that bars any vehicle with air brakes until you pass (383.95(a), 383.135(a)(2)).
What happens if I fail the combination vehicles test?
You cannot be issued a Class A permit or licence (383.135(a)(3)).
Can a Class A driver drive a Class B vehicle?
Yes, with the right endorsements. A Class A licence covers Class B and Class C vehicles, and a Class B covers Class C (383.91(c)).
Is there a separate test for Class B?
Class B takes the general knowledge test, plus air brakes if the vehicle has them, and any endorsement tests. There is no combination test for Class B.
Which endorsements can I get on a permit?
Only passenger (P), school bus (S) and tank vehicle (N) (383.93(a)(2)).
Sources
- 49 CFR Part 383 (eCFR) — §383.91 (vehicle groups), §383.93 (endorsements), §383.95 (restrictions), §383.111 (required knowledge), §383.135 (80% pass mark, failing air brakes or combination), §383.153 (endorsement and restriction codes), Subpart I (TSA approval for hazmat).
- 49 CFR Part 380 — entry-level driver training.
- AAMVA Model Commercial Driver’s License Manual — the basis of every state’s CDL manual.
