At a Glance: The Three CDL Knowledge Tests
| General Knowledge |
50 questions, 60 minutes, 80% to pass (40 correct). Covers safe driving, cargo, vehicle systems, hours of service, pre-trip inspection, and hazmat basics. |
| Air Brakes |
25 questions, no time limit, 80% to pass (20 correct). Required if your vehicle has air brakes — which includes most Class A tractor-trailers. |
| Combination Vehicles |
20 questions, no time limit, 80% to pass (16 correct). Covers coupling/uncoupling, trailer sway, and multi-trailer handling. |
| Passing score |
80% on all three. You must pass each test individually — a high score on one does not offset a low score on another. |
| Where you test |
At your state’s driver licensing office. Tests are administered on a computer. Bring your regular driver’s license, Social Security number, and medical certificate. |
| Free practice tests |
Take Roehl’s free CDL practice tests — six tests across all three subjects, with two versions each: one with feedback and one that simulates the real exam. |
Your CDL Study Plan: Week by Week
The written tests are easier to pass with a structured approach than by reading the manual cover to cover and hoping for the best. Here’s a four-week plan that works around a normal schedule.
Week 1 — Get the Foundation
Download your state’s CDL manual (link to your state below) and read Sections 1–5: Introduction, Driving Safely, Transporting Cargo, Transporting Passengers basics, and Vehicle Inspection. These sections cover about 60% of the General Knowledge test.
Week 2 — General Knowledge Deep Dive
Read Sections 2 and 3 again with note-taking focus on: stopping distance formulas, weight limits (80,000 lbs GVW, 20,000 lbs single axle, 34,000 lbs tandem), hours of service rules (11-hour driving limit, 14-hour window, 30-minute break after 8 hours), and pre-trip inspection sequence. Then take the General Knowledge V1 practice test with feedback turned on.
Week 3 — Air Brakes & Combination Vehicles
Air Brakes is the section most people underestimate. Read Section 5 (Air Brakes) twice — once for understanding, once to memorize the specific numbers: normal air pressure range (100–125 psi), low-pressure warning (60 psi), applied leakage rate (3 psi/minute single, 4 psi/minute combination). Then read Section 6 (Combination Vehicles) with focus on the coupling sequence and when trailer brakes should be used. Take both practice tests.
Week 4 — Simulate the Real Exam
Take all three Version 2 tests back to back with no feedback — timed, no notes, simulating the actual test environment. Review every question you missed and re-read the relevant manual section. Repeat until you’re consistently hitting 90%+ on all three. Then schedule your CLP appointment.
General Knowledge Test: What’s on It and What to Study
The General Knowledge test has 50 questions and covers everything a Class A driver needs to know before getting behind the wheel. It’s the broadest of the three tests and the one with the most material, but also the most predictable.
Topics covered
| Topic |
What to know |
| Safe operation |
Speed management, space management, visual search, night driving, adverse conditions, mountain driving. |
| Vehicle inspection (pre-trip) |
The full inspection sequence — engine compartment, cab interior, outside the vehicle, lights, and coupling devices. Know the sequence, not just the checklist. |
| Cargo handling |
Securing loads, weight distribution, legal weight limits (80,000 lbs GVW; 20,000 lbs single axle; 34,000 lbs tandem axle), center of gravity. |
| Hours of service |
11-hour driving limit, 14-hour on-duty window, 30-minute break after 8 hours, 10-hour off-duty requirement, 60/70-hour weekly limits. |
| Vehicle systems |
Brakes, steering, tires, suspension, fuel, electrical. Know what to check and what a defect looks like. |
| Hazardous materials basics |
Placards, shipping papers, what requires a hazmat endorsement. You’ll be tested on basics even without the HME. |
| Emergencies |
Brake failure, tire blowout, skids, fires, accident procedures. |
What to focus on — General Knowledge
- Stopping distance math. The test asks how many feet it takes to stop at a given speed. Memorize: at 55 mph, a fully loaded truck needs roughly 390 feet to stop. Most test-takers underestimate.
- Hours of service edge cases. The 14-hour rule applies even if you’re sitting still. The 30-minute break must happen before the 8-hour mark, not at 8 hours.
- Weight limits per axle, not just total. 80,000 lbs is the gross — but single axle max is 20,000 lbs and tandem is 34,000 lbs.
- Pre-trip inspection sequence. The test asks about specific steps and specific order. Study the sequence, not just the items.
Take the practice test: General Knowledge V1 (with feedback) • General Knowledge V2 (exam simulation)
Air Brakes Test: What’s on It and What to Study
The Air Brakes test has 25 questions and covers how air brake systems work, how to inspect them, and how to respond when something goes wrong. Most candidates underestimate this section — it’s more technical than the General Knowledge test and has the most specific numbers to memorize.
The numbers you must memorize
| Normal air pressure range |
100–125 psi |
| Low-pressure warning activates at |
60 psi (warning light and buzzer) |
| Applied leakage rate (single vehicle) |
No more than 3 psi/minute with brakes applied |
| Applied leakage rate (combination) |
No more than 4 psi/minute with brakes applied |
| Static leak rate (no brakes applied) |
No more than 2 psi/minute |
| Air pressure build time |
From 85 to 100 psi in 45 seconds or less at governed RPM |
| Brake lag time |
Air brakes take ½ second to respond after pedal application — factor into stopping distance |
| Spring brakes activate at |
Between 20–45 psi (parking brakes automatically engage) |
What to focus on — Air Brakes
- Confusing the low-pressure warning threshold (60 psi) with the spring brake activation range (20–45 psi). They are different numbers with different consequences.
- The difference between single and combination vehicle leakage rates. 3 psi for single, 4 psi for combination.
- Brake lag distance. On top of normal reaction time, air brakes add roughly 32 feet of travel at 55 mph before brakes engage.
- When to use the parking brake vs. the service brake vs. emergency brakes. The test distinguishes all three.
Take the practice test: Air Brakes V1 (with feedback) • Air Brakes V2 (exam simulation)
Combination Vehicles Test: What’s on It and What to Study
The Combination Vehicles test has 20 questions and covers the specific challenges of operating a tractor-trailer versus a single vehicle — coupling and uncoupling, trailer sway, off-tracking, and multi-trailer handling.
| Topic |
What to know |
| Coupling and uncoupling |
The exact sequence for coupling a tractor to a trailer and uncoupling it safely. The test asks about specific steps and the consequences of skipping them. |
| Trailer sway |
What causes it, how to recognize it early, and the correct response (never brake — accelerate slightly to pull the trailer straight, then gradually reduce speed). |
| Off-tracking |
Rear wheels of a trailer track inside the path of the front wheels on curves. Relevant for clearance, turning, and safety. |
| Pintle hooks and converter dollies |
Used in double/triple trailer configurations. The test covers basic operation and safety checks. |
| Air lines and glad hands |
Service and emergency air lines, how to connect them, and what happens if an air line disconnects while driving. |
What to focus on — Combination Vehicles
- The trailer sway response. The instinct is to brake — the correct answer is to accelerate slightly, then ease off. Most wrong answers choose braking.
- The coupling sequence steps, in order. The test often presents the steps out of order and asks which is correct.
- Converter dolly safety checks. Less familiar to new drivers, but the test includes them.
Take the practice test: Combination Vehicles V1 (with feedback) • Combination Vehicles V2 (exam simulation)
Find Your State’s CDL Manual
Every state publishes a CDL manual based on the federal FMCSA model. The questions on your state’s exam come from this manual, so it’s the primary study source. Start here before taking any practice tests. Find your state’s manual at the links below — these are direct links to the official PDF from each state’s licensing agency.
State manual links: AL • AK • AZ • AR • CA • CO • CT • DE • FL • GA • HI • ID • IL • IN • IA • KS • KY • LA • ME • MD • MA • MI • MN • MS • MO • MT • NE • NV • NH • NJ • NM • NY • NC • ND • OH • OK • OR • PA • RI • SC • SD • TN • TX • UT • VT • VA • WA • WV • WI • WY
After You Pass: What Comes Next
Passing all three knowledge tests earns you your Commercial Learner’s Permit (CLP). You must hold the CLP for at least 14 days before you’re eligible to take the CDL skills test — the behind-the-wheel exam that’s the final step to a full CDL.
One way to skip paying for CDL school while training: Roehl’s Get Your CDL™ program hires you as a full-time employee before you have your CDL. You use Roehl’s practice tests to prepare, come in with your CLP, and earn a paycheck throughout your three-week training. See the full process in our step-by-step CDL guide.
Frequently Asked Questions: Studying for the CDL Test
How hard is the CDL written test?
It’s straightforward if you study the right material. Most candidates who fail do so on Air Brakes — it’s the most technical section and has the most specific numbers to memorize. Following a structured study plan and using practice tests before your appointment significantly improves pass rates.
How many questions are on the CDL written test?
There are three separate tests: General Knowledge (50 questions), Air Brakes (25 questions), and Combination Vehicles (20 questions). You must pass all three to receive your CLP. Passing score is 80% on each.
What is the CDL written test passing score?
80% on each test, taken individually. General Knowledge: 40 of 50 correct. Air Brakes: 20 of 25. Combination Vehicles: 16 of 20.
How long does the CDL written test take?
The General Knowledge test allows 60 minutes. The Air Brakes and Combination Vehicle tests have no time limit. Most test-takers finish all three in two to three hours.
Can I take the CDL written test online?
No — all CDL knowledge tests must be taken in person at a state driver licensing office. You can use online practice tests to prepare, but the official exam is always at the DMV.
What should I study first for the CDL test?
Start with your state’s CDL manual — especially Sections 2 (Driving Safely), 3 (Transporting Cargo), and 5 (Air Brakes). Then use practice tests to identify gaps. Most people benefit from studying Air Brakes last so the specific numbers are freshest going into the exam.
Are CDL practice tests accurate?
Roehl’s practice tests are drawn from real CDL exam questions based on the federal FMCSA manual that all states use. The questions match the format, difficulty, and subject distribution you’ll see at the DMV.
What is the hardest part of the CDL written test?
Air Brakes is consistently the section with the highest fail rate. It requires memorizing specific pressure values, leakage rates, and brake response times rather than general concepts. The Combination Vehicles test is also tricky for candidates who focus on memorizing steps but don’t understand the reasoning behind them.
How many times can I retake the CDL written test?
This varies by state. Most states allow retakes after a waiting period (usually one business day to a week). Check with your state’s licensing agency for specifics.
Do I need to study for CDL endorsements before I apply for my first job?
For most starting positions at Roehl, you need the Air Brakes and Combination Vehicle endorsements — which are included in the three-test CLP process above. The Hazmat endorsement is worth adding later; it opens up more freight options and increases earning potential, but isn’t required on day one.
Ready to put your study to work? Take the free CDL practice tests or find out how to get paid while you get your CDL.
