FREE CDL Study Guide 2026: General Knowledge Test

This free CDL study guide teaches the General Knowledge test — the written exam every applicant for a Commercial Driver License must pass, no matter which class they want.^[1] It is built directly from the AAMVA CDL Manual, the model that states base their tests on.

It’s interactive, not a wall of text: every module has a built-in checkpoint quiz, hover-able glossary terms, labeled diagrams, and concept questions, so you learn by doing. We cover the General Knowledge content thoroughly and summarize the endorsement tests (Air Brakes, Combination Vehicles, Hazmat, Passenger/School Bus, Tank, Doubles/Triples) so you know exactly what each adds.

Work through the five modules, test yourself at each checkpoint, then round out your free CDL prep with our practice test and flashcards.

CDL Exam Snapshot

Because the CDL is a federal-state partnership, the rules a commercial driver must know are uniform across the country — set by the FMCSA — but the actual test is given by your state licensing agency, which is why the exact question count and passing percentage vary slightly from state to state.^[2] This guide teaches the federal/AAMVA standard, organized into five study modules:

The biggest share of the test is safe driving — control, speed, space, and hazards — so that is where you should spend most of your study time. The chart shows the rough emphasis we use to organize this guide; it is a teaching aid, not an official blueprint, since the AAMVA manual presents the material as topics rather than weighted domains.

1 · CDL Basics, Classes & Disqualification

Before the driving content, the test checks that you understand what license you need, what it lets you do, and what can take it away. These federal rules are uniform nationwide.^[2]

Classes A, B, C & the CMV definition

The three CDL classes are defined by vehicle weight ratings and what is being towed, not by the cargo or the job.^[2] A vehicle is a commercial motor vehicle if it meets a weight threshold, carries 16 or more passengers including the driver, or hauls placarded hazardous materials.

Endorsements, restrictions & ELDT

An endorsement expands what you may operate; a restriction limits it. Each endorsement requires its own knowledge test (and sometimes a skills test). The most common restriction is the “L”restriction, added when you don’t pass the air-brake test or test in a vehicle without air brakes — it bars you from air-brake vehicles entirely.

Two tests come up for many drivers even though they aren’t “endorsements” in the usual sense. The Air Brakes test is required to drive any air-brake CMV, and the Combination Vehicles test is required for a Class A CDL (it covers coupling and uncoupling, the fifth wheel, and off-tracking). Since February 7, 2022, the Entry-Level Driver Training rule also requires new applicants for a first Class A or B CDL — or for the Hazmat, Passenger, or School Bus endorsement — to complete training from a provider on the FMCSA Training Provider Registry before testing.^[3]

Disqualifications & medical rules

A commercial driver is held to a stricter standard than a passenger-car driver. The legal blood alcohol limit is 0.04% — half the 0.08% limit for regular drivers — and a first offense brings a one-year disqualification (three years if hauling hazmat).^[4] By holding a CDL you give implied consent to testing, so refusing a required test carries the same penalty as failing it.

2 · Vehicle Inspection

You are responsible for the safety of your vehicle. Federal and state law require inspections, the CDL skills test includes a graded vehicle-inspection portion, and a thorough inspection prevents breakdowns and crashes.^[1]

The seven-step inspection method

The AAMVA manual teaches a routine seven-step method so nothing gets missed. You start with a general overview and finish by testing the brakes:

What and when to inspect

There are three kinds of inspection: pre-trip (before you drive), en-route (watch your gauges and stop to check tires, brakes, and cargo), and post-trip (after the trip, with a written report of any defects). If any inspection finds a defect that affects safe operation, you must not driveuntil it’s repaired.

3 · Safe Driving & Vehicle Control

This is the heart of the General Knowledge test. Controlling a heavy vehicle is different from driving a car: it takes longer to stop, needs more room, and demands more deliberate steering and shifting.^[1]

Basic control, steering & shifting

Hold the wheel firmly with both hands so a pothole or curb can’t jerk it from your grip. To shift a manual transmission smoothly, drivers double clutch: release the accelerator, push in the clutch and shift to neutral, release the clutch, let the engine speed match the road speed, then push in the clutch again and shift to the next gear. Shift up when the tachometer reaches the top of the engine’s recommended range.

Seeing & communicating

Look far enough ahead — about 12 to 15 seconds, a quarter mile at highway speed — so you have time to react. Use your mirrors regularly; a convex (spot) mirror gives a wider view of the sides, though objects in it look smaller and farther away than they are. To communicate, signal turns and lane changes early and keep the signal on until the maneuver is complete, and use your horn, brake lights, and flashers to be seen.

Controlling speed & stopping distance

A heavy vehicle’s total stopping distance is made of three parts — perception distance, reaction distance, and braking distance — and at 55 mph it adds up to more than 400 feet, longer than a football field.

The most important thing to internalize: braking distance grows with the square of speed. Double your speed and the braking distance roughly quadruples.

That’s why you slow before a curve (braking in a curve can cause a skid), and why a loaded truck or one with a high center of gravity can roll over even at a posted curve speed set for cars. On slick roads, reduce speed because traction — the grip between tires and road — is what you rely on to steer and stop.

Managing space

The space ahead is the one you control most, so keep a safe following distance: at least one second for every 10 feet of vehicle length below 40 mph, plus one extra second above 40 mph.

Also manage the space behind(don’t let drivers tailgate; ease off gradually), to the sides (don’t travel beside others in your blind spots), overhead (know your height for bridges and signs), and below (watch for low clearance at railroad crossings and steep driveways).

4 · Seeing Hazards & Emergencies

The test rewards drivers who spot trouble early and know what to do when something goes wrong. Seeing a hazard in time turns an emergency into a routine adjustment.^[1]

Seeing hazards & distraction

A hazard is any road condition or other road user that is a possible danger. Scan for clues — a ball rolling into the road, a car drifting in its lane, brake lights far ahead — so you have time to slow or steer around it.

Distracted driving (anything that takes your eyes, hands, or mind off the road) is a leading cause of crashes; texting and handheld phone use in a CMV are banned by federal rule and count as serious violations. Aggressive driving and road rageare dangerous too — stay out of an angry driver’s way and don’t engage.

Night, fog, winter & mountain driving

At night, never drive faster than the range your headlights light up — low beams reach about 250 feet and high beams about 350 to 500 feet — or you can’t stop within the distance you can see (dim your high beams within 500 feet of oncoming traffic). In fog, slow down (or wait it out) and use low beams and fog lights; high beams reflect back and cut visibility.

In winter, watch for black ice — a thin, clear layer that makes the road just look wet — and reduce speed for snow and ice. On long mountain downgrades, pick a safe low gear before you start down and let the engine and retarder do the braking; if your brakes fail, use an escape ramp.

Skids, ABS, fires & railroad crossings

A jackknife happens when the trailer wheels lock during hard braking and the trailer swings toward the tractor. Antilock brakes help prevent it by keeping the wheels from locking — so you keep steering control — and on an ABS vehicle you brake normally with steady pressure rather than pumping. If you start to hydroplane, ease off the accelerator and let the vehicle slow.

Know your fire classes: Class A (wood, paper), Class B (fuel, oil), and Class C (electrical) — aim the extinguisher at the base of the fire. At a railroad crossing, never start across unless there’s room to clear the tracks completely without stopping; at an unguarded crossing, slow down, look and listen, and be ready to stop. After a crash, protect the scene(warning triangles behind the vehicle), notify authorities, and care for the injured without moving them unless there’s danger like fire.

5 · Cargo, Weight & Securement

Even drivers who never load their own truck are responsible for inspecting cargo and recognizing overloads and poor weight distribution.^[5] The General Knowledge test covers the basics; tank and hazmat specifics live in those endorsements.

Weights: GVWR, GCWR & axle weight

GVWR is the manufacturer’s maximum for a single vehicle plus load; GCWR is the maximum for a power unit plus its trailer(s) and load. The actual gross vehicle weight must stay at or below the rating and within legal axle limits. A high center of gravity makes a vehicle tip more easily, so load heavy items low and spread the weight evenly.

Securing the load

Cargo must be secured so it can’t shift, fall, or leak. Use blocking and bracing to keep it from sliding and tipping, and tiedowns rated for the load. The rules: at least one tiedown per 10 feet of cargo, always a minimum of two, and the combined working load limit of all tiedowns must be at least halfthe cargo’s weight.

How to Use This Study Guide

A study guide is a map, not the whole territory — use it alongside your state’s official CDL manual and our free tools. Work the modules in order, because the safe-driving content (modules 3 and 4) is the largest part of the test.

CDL Glossary

Quick definitions for the terms you’ll see most across the CDL General Knowledge test:

AAMVA CDL Manual

The Model Commercial Driver License Manual published by the American Association of Motor Vehicle Administrators. States base their CDL knowledge tests on it, so it is the source material for the General Knowledge test.

ABS

Antilock braking system — keeps the wheels from locking during hard braking so the driver keeps steering control. On an ABS vehicle you brake normally with steady, firm pressure rather than pumping.

Axle weight

The weight transmitted to the ground by one axle or one set of axles. Federal and state limits cap how much a single axle or tandem may carry, separate from the total gross weight.

Black ice

A thin, clear layer of ice that lets the dark pavement show through, so the road looks merely wet. It is dangerous because it is nearly invisible and drivers do not slow for it.

Blocking and bracing

Securement methods: blocking is wedged around cargo to keep it from sliding, and bracing extends from the cargo to a structure to keep it from tipping.

Braking distance

How far a vehicle travels after the brakes are applied until it stops. It grows with the square of speed, so doubling speed roughly quadruples it.

CDL

Commercial Driver License — the license required to operate a commercial motor vehicle. It is issued by your state of domicile and printed with your class plus any endorsement and restriction codes.

Class A CDL

Required for a combination of vehicles with a gross combination weight rating of 26,001 lb or more where the towed unit is rated over 10,000 lb — for example, a tractor-trailer.

Class B CDL

Required for a single vehicle with a gross vehicle weight rating of 26,001 lb or more, or such a vehicle towing a unit not over 10,000 lb — for example, a straight truck or large bus.

Class C CDL

Required for a vehicle that does not meet Class A or B but is designed to carry 16 or more passengers (including the driver) or to transport placarded hazardous materials.

CMV

Commercial motor vehicle — a vehicle that meets a CDL weight threshold (26,001+ lb GVWR or GCWR), is designed to carry 16 or more passengers including the driver, or transports placarded hazardous materials.

ELDT

Entry-Level Driver Training — the FMCSA requirement (effective February 7, 2022) that new applicants for a first Class A or B CDL, or the Hazmat, Passenger, or School Bus endorsement, complete training from a registered provider before testing.

Endorsement

An added authority printed on a CDL that lets you operate a special vehicle or carry special cargo (such as Hazmat, Tank, Passenger, or School Bus), earned by passing an extra knowledge — and sometimes skills — test.

Escape ramp

A truck-escape lane built on steep downgrades to safely stop a vehicle whose brakes have failed. If your brakes fail on a grade, use the escape ramp rather than passing it.

FMCSA

The Federal Motor Carrier Safety Administration — the U.S. Department of Transportation agency that sets the federal rules for commercial drivers, including disqualifications, medical standards, and Entry-Level Driver Training.

Following distance

The space a driver keeps behind the vehicle ahead. The basic rule: one second for every 10 feet of vehicle length under 40 mph, plus one extra second above 40 mph.

GCWR

Gross combination weight rating — the manufacturer's maximum weight for a power unit plus its trailer(s) and load.

General Knowledge test

The written CDL knowledge test that every applicant must pass, regardless of class. It is about 50 multiple-choice questions, usually requiring around 80% to pass, drawn from the AAMVA CDL Manual's safe-driving content.

GVW

Gross vehicle weight — the actual total weight of the vehicle and its load right now, which must stay at or below the GVWR and within legal axle limits.

GVWR

Gross vehicle weight rating — the maximum weight of a single vehicle plus its load that the manufacturer specifies. The 26,001 lb GVWR figure separates the CDL classes.

Hydroplaning

When the tires ride up on a film of water and lose contact with the road, so steering and braking traction is lost. Recover by easing off the accelerator and letting the vehicle slow.

Jackknife

When the trailer swings around toward the tractor at a sharp angle, like a folding pocketknife — usually caused by the trailer wheels locking during hard braking on a slippery surface.

Medical examiner's certificate

A document proving a commercial driver meets the DOT physical-qualification standards. The driver must carry it and keep it current to remain medically qualified.

Out-of-service order

A roadside order declaring a driver or vehicle unsafe to operate until the listed defects are fixed or the disqualifying condition ends. Operating in violation of it is itself a serious offense.

Perception distance

How far a vehicle travels from the moment the driver's eyes see a hazard until the brain recognizes it — about 1.75 seconds for an alert driver, roughly 142 feet at 55 mph.

Reaction distance

How far a vehicle travels while the driver moves a foot from the accelerator to the brake pedal — about 0.75 second, roughly 61 feet at 55 mph.

Restriction

A code on a CDL that limits what the driver may operate — the opposite of an endorsement. The most common is the 'L' restriction, which bars vehicles equipped with air brakes.

Total stopping distance

The full distance a vehicle travels from the moment a driver sees a hazard until the vehicle stops, equal to perception distance plus reaction distance plus braking distance.

Working load limit

The maximum load a tiedown or securement device is rated to hold under normal use, marked by the manufacturer. The combined limit of all tiedowns must be at least half the cargo weight.

Free CDL Study Materials & Resources

Everything you need to prepare for the CDL is free here — no paywall, no sign-up. This guide is the foundation; pair it with the rest of our free CDL study materials for active recall, timed practice, and last-minute review:

• CDL Practice Test — exam-style General Knowledge questions, with explanations.
• CDL Flashcards — active-recall decks for the rules, stopping distances, hazards, and cargo facts.

CDL Study Guide FAQ

References

1. 1.American Association of Motor Vehicle Administrators. “Commercial Driver License — Model CDL Manual.” AAMVA. ↑
2. 2.Federal Motor Carrier Safety Administration. “Commercial Driver's License Program.” FMCSA (U.S. Department of Transportation). ↑
3. 3.Federal Motor Carrier Safety Administration. “Entry-Level Driver Training (ELDT).” FMCSA (U.S. Department of Transportation). ↑
4. 4.Federal Motor Carrier Safety Administration. “CDL Disqualifications.” FMCSA (U.S. Department of Transportation). ↑
5. 5.Federal Motor Carrier Safety Administration. “Cargo Securement Rules.” FMCSA (U.S. Department of Transportation). ↑

Sources for the concept answers

Every answer in the CDL concept questions above is drawn from an official primary source:

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