CDL combination vehicles practice test
40 practice questions written from the AAMVA model Commercial Driver's License Manual (March 2025 edition). The mock test asks 20 questions and needs 16 correct, the federal 80% pass share.
Practice questions written from the AAMVA model CDL manual; not affiliated with FMCSA, AAMVA or any state licensing agency.
examprepare.help wrote this study aid. AAMVA, FMCSA and the state agencies did not write or review it, and your real test may differ.
Practice shows the right answer and the manual page after every question.The mock test asks 20 questions and needs 16 correct to pass.
The real combination vehicles test
Federal rules require at least 80% correct on every CDL knowledge test (49 CFR 383.135(b)(1)). On a 20-question test that is 16 correct.
Test sizes vary by state. Twenty questions is the common size of the combination vehicles test. Each state sets its own test, so your test may ask more or fewer. The 80% rule is the same in every state. Check your state licensing agency before your test.
Rule source: 49 CFR 383.135(b)(1) (80 percent to pass a CDL knowledge test).
Who takes this test
You take the combination vehicles test if you want to drive combination vehicles, such as a tractor-trailer. A Class A license covers combination vehicles.
Manual section 1.1.1, Knowledge Tests, page 1-5. Class A definition: Section 1, page 1-1. Our questions come from Section 6 of the manual.
The CDL manual
Commercial Driver's License Manual, Modernized Testing System
AAMVA writes this model manual with support from the Federal Motor Carrier Safety Administration (FMCSA). Most states use it as their CDL manual. Our copy is the PDF that Service Oklahoma publishes.
Read the manual (PDF, Service Oklahoma) or the CDL manual page on the FMCSA site.
We read this edition on 2026-10-05. Your state may print its own edition. Check that your copy is the current one.
Key numbers from the manual
| Rule | Value | PDF page |
|---|---|---|
| Rollover risk of a fully loaded rig | Ten times more likely to roll over in a crash than an empty rig | 82 |
| Rearward amplification of triples | 3.5 times as easy to roll the last trailer as a five-axle tractor | 82 |
| Pressure at which the tractor protection valve closes | 20 to 45 psi | 86 |
| Air-line color code | Blue for service, red for emergency | 86 |
| Shut-off valve positions | Open except at the back of the last trailer, which is closed | 87 |
| ABS required on trailers and converter dollies | Built on or after March 1, 1998 | 87 |
| Space between the upper and lower fifth wheel | No space at all | 89 |
Questions about the combination vehicles test
How many questions are on the CDL combination vehicles test?
Twenty questions is the common size of the combination vehicles test. Each state sets its own test, so your test may ask more or fewer. Our mock test asks 20.
What score do I need to pass the CDL combination vehicles test?
Federal rules require at least 80% correct on every CDL knowledge test (49 CFR 383.135(b)(1)). On a 20-question test that is 16 correct. Our mock test needs 16 of 20.
Who has to take the combination vehicles test?
You take the combination vehicles test if you want to drive combination vehicles, such as a tractor-trailer. A Class A license covers combination vehicles. Manual section 1.1.1, Knowledge Tests, page 1-5. Class A definition: Section 1, page 1-1.
Are these the real CDL test questions?
No. examprepare.help wrote these 40 practice questions from the AAMVA model Commercial Driver's License Manual (March 2025 edition). They are not the real test questions. Practice questions written from the AAMVA model CDL manual; not affiliated with FMCSA, AAMVA or any state licensing agency. Check your state licensing agency before you take the real test.
Which manual are the questions from?
The AAMVA model Commercial Driver's License Manual (March 2025 edition). AAMVA writes this model manual with support from FMCSA, and most states use it as their CDL manual. Every answer cites the manual section and the printed page.
All 40 questions and answers
Read the bank before a mock test. Each answer names the manual section and the printed page.
- 1.How much more likely is a fully loaded rig to roll over in a crash than an empty rig?
6.1.1 – Rollover Risks
- Ten times more likely
More than half of truck driver deaths in crashes are the result of rollovers, and fully loaded rigs are ten times more likely to roll over than empty rigs. Manual: 6.1.1 – Rollover Risks, page 6-1.
- 2.Which two things help prevent a rollover in a combination vehicle?
6.1.1 – Rollover Risks
- Keep cargo as close to the ground as possible and drive slowly around turns
Keep the cargo as close to the ground as possible and drive slowly around corners, ramps, and off ramps to prevent rollover. Manual: 6.1.1 – Rollover Risks, page 6-1.
- 3.What does a rearward amplification of 3.5 for triples mean?
6.1.2 – Steer Gently
- The last trailer can be rolled 3.5 times as easily as a five-axle tractor
Rearward amplification of 3.5 for triples means you can roll the last trailer of triples 3.5 times as easily as a five-axle tractor. Manual: 6.1.2 – Steer Gently, page 6-1.
- 4.Why must you steer gently and smoothly when pulling trailers?
6.1.2 – Steer Gently
- A sudden movement can cause the crack-the-whip effect and tip the trailer over
Trucks with trailers have a dangerous crack-the-whip effect, and a quick lane change can turn the trailer over, so steer gently and smoothly. Manual: 6.1.2 – Steer Gently, page 6-1.
- 5.How does an empty combination vehicle compare with a fully loaded one when stopping?
6.1.3 – Brake Early
- It takes longer to stop when empty than when fully loaded
Large combination vehicles take longer to stop when they are empty than when they are fully loaded, because stiff springs and strong brakes give poor traction. Manual: 6.1.3 – Brake Early, page 6-1.
- 6.When is a trailer jackknife most likely to happen?
6.1.5 – Prevent Trailer Skids
- When the trailer is empty or lightly loaded
When the wheels of a trailer lock up, the trailer tends to swing around, and this is more likely when the trailer is empty or lightly loaded. Manual: 6.1.5 – Prevent Trailer Skids, page 6-2.
- 7.What is the earliest and best way to recognize that the trailer has started to skid?
6.1.5 – Prevent Trailer Skids
- See it in your mirrors
The earliest and best way to recognize that the trailer has started to skid is by seeing it in your mirrors, so check them any time you brake hard. Manual: 6.1.5 – Prevent Trailer Skids, page 6-2.
- 8.How should you stop a trailer skid?
6.1.5 – Prevent Trailer Skids
- Release the brakes to get traction back
Release the brakes to get traction back; do not use the trailer hand brake to straighten the rig, because the trailer brakes caused the skid in the first place. Manual: 6.1.5 – Prevent Trailer Skids, page 6-3.
- 9.What is off-tracking?
6.1.6 – Turn Wide
- The rear wheels following a different path than the front wheels
Off-tracking, or cheating, is when the rear wheels follow a different path than the front wheels, and longer vehicles off-track more. Manual: 6.1.6 – Turn Wide, page 6-3.
- 10.If you cannot complete a right turn without entering another lane, when should you turn wide?
6.1.6 – Turn Wide
- As you complete the turn, keeping the rear of the vehicle close to the curb
Turn wide as you complete the turn rather than swinging wide to the left before starting, because that keeps other drivers from passing you on the right. Manual: 6.1.6 – Turn Wide, page 6-3.
- 11.When backing a trailer, which way do you turn the steering wheel?
6.1.7 – Backing with a Trailer
- In the opposite direction from where you want the trailer to go
When backing a trailer, you turn the steering wheel in the opposite direction, and once the trailer starts to turn you must turn the wheel the other way to follow it. Manual: 6.1.7 – Backing with a Trailer, page 6-3.
- 12.When should you use the trailer hand valve?
6.2.1 – Trailer Hand Valve
- Only to test the trailer brakes
The trailer hand valve works the trailer brakes and should be used only to test the trailer brakes, not in driving, because it can make the trailer skid. Manual: 6.2.1 – Trailer Hand Valve, page 6-4.
- 13.Why should you never use the trailer hand valve for parking?
6.2.1 – Trailer Hand Valve
- All the air might leak out and unlock the brakes
Never use the hand valve for parking because all the air might leak out and unlock the brakes on trailers that do not have spring brakes; use the parking brakes or chocks. Manual: 6.2.1 – Trailer Hand Valve, page 6-4.
- 14.What does the tractor protection valve do?
6.2.2 – Tractor Protection Valve
- It keeps air in the tractor brake system if the trailer breaks away or develops a leak
The tractor protection valve keeps air in the tractor or truck brake system should the trailer break away or develop a bad leak. Manual: 6.2.2 – Tractor Protection Valve, page 6-5.
- 15.At what pressure does the tractor protection valve close automatically?
6.2.2 – Tractor Protection Valve
- 20 to 45 psi
The tractor protection valve closes automatically if air pressure is low, in the range of 20 to 45 psi, which puts on the trailer emergency brakes. Manual: 6.2.2 – Tractor Protection Valve, page 6-5.
- 16.What does pushing in the trailer air supply control do?
6.2.3 – Trailer Air Supply Control
- Supplies the trailer with air
On newer vehicles the trailer air supply control is a red eight-sided knob; push it in to supply the trailer with air and pull it out to shut the air off and put on the trailer emergency brakes. Manual: 6.2.3 – Trailer Air Supply Control, page 6-5.
- 17.What are the two air-lines on every combination vehicle?
6.2.4 – Trailer Air-lines
- The service line and the emergency line
Every combination vehicle has two air-lines, the service line and the emergency line, running between each vehicle. Manual: 6.2.4 – Trailer Air-lines, page 6-5.
- 18.What does the service air-line do?
6.2.4 – Trailer Air-lines
- It carries air controlled by the foot brake or trailer hand brake
The service line, also called the control or signal line, carries air controlled by the foot brake or the trailer hand brake and is connected to relay valves. Manual: 6.2.4 – Trailer Air-lines, page 6-5.
- 19.What are the two purposes of the emergency air-line?
6.2.4 – Trailer Air-lines
- It supplies air to the trailer air tanks and controls the trailer emergency brakes
The emergency line, also called the supply line, supplies air to the trailer air tanks and controls the emergency brakes on combination vehicles. Manual: 6.2.4 – Trailer Air-lines, page 6-5.
- 20.Which colors are commonly used for the service and emergency air-lines?
6.2.4 – Trailer Air-lines
- Blue for service and red for emergency
Colors are sometimes used to avoid mistakes: blue is used for the service lines and red for the emergency supply lines. Manual: 6.2.4 – Trailer Air-lines, page 6-5.
- 21.How should you connect glad hands?
6.2.5 – Hose Couplers (Glad Hands)
- Press the two seals together with the couplers at a 90 degree angle and turn to lock
Clean the couplers and rubber seals, press the seals together with the couplers at a 90 degree angle to each other, and turn to join and lock them. Manual: 6.2.5 – Hose Couplers (Glad Hands), page 6-5.
- 22.Which shut-off valves should be closed on a combination vehicle?
6.2.7 – Shut-off Valves
- Only the valves at the back of the last trailer
All shut-off valves must be open except the ones at the back of the last trailer, which must be closed. Manual: 6.2.7 – Shut-off Valves, page 6-6.
- 23.Why must you use wheel chocks when you park a trailer that has no spring brakes?
6.2.8 – Trailer Service, Parking and Emergency Brakes
- The emergency brakes hold only as long as air remains in the trailer air tank
Trailers without spring brakes have emergency brakes that work from air stored in the trailer air tank; when the air leaks away there are no brakes, so use wheel chocks. Manual: 6.2.8 – Trailer Service, Parking and Emergency Brakes, page 6-6.
- 24.Which trailers and converter dollies are required to have ABS?
6.3.1 – Trailers Required to Have ABS
- Those built on or after March 1, 1998
All trailers and converter dollies built on or after March 1, 1998 are required to have ABS, though many older ones were voluntarily equipped. Manual: 6.3.1 – Trailers Required to Have ABS, page 6-6.
- 25.How much control do you gain if only the trailer has ABS?
6.3.2 – Braking with ABS
- You still get more control and the trailer is less likely to swing out
Having ABS on only the trailer, or even on only one axle, still gives you more control over the vehicle during braking. Manual: 6.3.2 – Braking with ABS, page 6-7.
- 26.When inspecting the fifth wheel before coupling, how should the wheel be positioned?
6.4.1 – Coupling Tractor-Semitrailers
- Tilted down toward the rear of the tractor, with the jaws open
Check that the fifth wheel is in proper position for coupling: wheel tilted down toward the rear of the tractor, jaws open, and safety unlocking handle in the automatic lock position. Manual: 6.4.1 – Coupling Tractor-Semitrailers, page 6-7.
- 27.Why should you never back under the trailer at an angle?
6.4.1 – Coupling Tractor-Semitrailers
- You might push the trailer sideways and break the landing gear
Put the tractor directly in front of the trailer; never back under at an angle because you might push the trailer sideways and break the landing gear. Manual: 6.4.1 – Coupling Tractor-Semitrailers, page 6-8.
- 28.How do you know the trailer height is correct before coupling?
6.4.1 – Coupling Tractor-Semitrailers
- The trailer is low enough to be raised slightly by the tractor when backed under
The trailer should be low enough that it is raised slightly by the tractor when the tractor is backed under it; if it is too low the tractor may damage the nose, and if too high it may not couple. Manual: 6.4.1 – Coupling Tractor-Semitrailers, page 6-8.
- 29.After coupling, how much space should there be between the upper and lower fifth wheel?
6.4.1 – Coupling Tractor-Semitrailers
- No space at all
Make sure there is no space between the upper and lower fifth wheel; if there is space, the kingpin may be on top of the closed fifth wheel jaws and the trailer could come loose. Manual: 6.4.1 – Coupling Tractor-Semitrailers, page 6-8.
- 30.How do you check that the trailer is locked onto the tractor after coupling?
6.4.1 – Coupling Tractor-Semitrailers
- Raise the landing gear slightly and pull gently forward with the trailer brakes locked
Raise the trailer landing gear slightly off the ground and pull the tractor gently forward while the trailer brakes are still locked to check that the trailer is locked onto the tractor. Manual: 6.4.1 – Coupling Tractor-Semitrailers, page 6-8.
- 31.When you inspect the coupling by looking into the back of the fifth wheel, what should the jaws be around?
6.4.1 – Coupling Tractor-Semitrailers
- The shank of the kingpin
Look into the back of the fifth wheel and make sure the jaws have closed around the shank of the kingpin, not the head. Manual: 6.4.1 – Coupling Tractor-Semitrailers, page 6-8.
- 32.How far should you raise the landing gear after coupling?
6.4.1 – Coupling Tractor-Semitrailers
- All the way up, and secure the crank handle
Raise the landing gear all the way up and secure the crank handle; never drive with the landing gear only part way up, because it may catch on railroad tracks or other things. Manual: 6.4.1 – Coupling Tractor-Semitrailers, page 6-9.
- 33.How do you ease the pressure on the fifth wheel locking jaws before uncoupling?
6.4.2 – Uncoupling Tractor-Semitrailers
- Shut off the trailer air supply to lock the trailer brakes and back up gently
Shut off the trailer air supply to lock the trailer brakes, then ease pressure on the fifth wheel locking jaws by backing up gently to help release the locking lever. Manual: 6.4.2 – Uncoupling Tractor-Semitrailers, page 6-9.
- 34.How much should you turn the landing gear crank on a loaded trailer?
6.4.2 – Uncoupling Tractor-Semitrailers
- A few extra turns after firm contact to lift some weight, but not off the fifth wheel
If the trailer is loaded, after the landing gear makes firm contact with the ground, turn the crank a few extra turns to lift some weight off the tractor, but do not lift the trailer off the fifth wheel. Manual: 6.4.2 – Uncoupling Tractor-Semitrailers, page 6-9.
- 35.What should you do with the air-lines and electrical cable after uncoupling?
6.4.2 – Uncoupling Tractor-Semitrailers
- Connect the glad hands to dummy couplers and hang the electrical plug down
Connect the air-line glad hands to dummy couplers or couple them together, and hang the electrical cable with the plug down to prevent moisture from entering it. Manual: 6.4.2 – Uncoupling Tractor-Semitrailers, page 6-9.
- 36.Why should the fifth wheel not be too far forward?
6.5.1 – Additional Things to Check during a Walk-around Inspection
- The tractor frame could hit the landing gear or the cab could hit the trailer during turns
Check that the fifth wheel is not so far forward that the tractor frame will hit the landing gear, or the cab hit the trailer, during turns. Manual: 6.5.1 – Additional Things to Check during a Walk-around Inspection, page 6-13.
- 37.How do you check that air flows to all trailers?
6.5.2 – Combination Vehicle Brake Check
- Open the emergency line shut-off at the rear of the last trailer and listen for escaping air
Open the emergency line shut-off valve at the rear of the last trailer; you should hear air escaping, showing the entire system is charged. Manual: 6.5.2 – Combination Vehicle Brake Check, page 6-13.
- 38.How do you test the tractor protection valve?
6.5.2 – Combination Vehicle Brake Check
- Reduce the air pressure and check that the trailer air supply control pops out at 20 to 45 psi
With the system charged and the engine off, step on and off the brake pedal to reduce pressure; the trailer air supply control should pop out, usually within 20 to 45 psi. Manual: 6.5.2 – Combination Vehicle Brake Check, page 6-13.
- 39.How do you test the trailer emergency brakes?
6.5.2 – Combination Vehicle Brake Check
- Charge the system, pull out the trailer air supply control, and pull gently with the tractor
Charge the trailer air brake system and check that the trailer rolls freely, then pull out the trailer air supply control and pull gently with the tractor to check the emergency brakes are on. Manual: 6.5.2 – Combination Vehicle Brake Check, page 6-14.
- 40.How do you test the trailer service brakes?
6.5.2 – Combination Vehicle Brake Check
- Move forward slowly and apply the trailer brakes with the hand control
With normal air pressure, release the parking brakes, move the vehicle forward slowly, and apply the trailer brakes with the hand control, if so equipped; you should feel the brakes come on. Manual: 6.5.2 – Combination Vehicle Brake Check, page 6-14.