247SimpleTests
CDL · Shifting Gears

What does 'lugging' an engine mean and why is it harmful?

Reviewed by Jordan Morley, Editorial Lead at 247SimpleTests. Written by the 247SimpleTests Editorial Team from the official source material cited on every question.

Correct answer

Operating the engine at too low an RPM under load, which damages the engine

  1. A Failing to use the clutch when shifting gears
  2. B Operating the engine at too low an RPM under load, which damages the engine
  3. C Running the engine at too high an RPM, which causes overheating
  4. D Carrying a load above the rated capacity of the truck

Why this is the answer

Lugging is operating the engine at too low an RPM for the load being carried, typically because the driver is in too high a gear. Symptoms include the engine straining, the truck slowing, and visible black smoke. Lugging damages the engine over time through excessive heat and stress on the rods, bearings, and head gasket. It also reduces fuel efficiency despite the lower RPM. The fix is to downshift one or more gears to bring engine RPM back into its normal operating range. Modern electronic engines often have alarms or computer protection that limit damage, but the right habit is to anticipate hills and load conditions and downshift before the engine starts to lug.
Source: FMCSA CDL Manual 2.3.4 Knowing When to Shift

Practice more questions

This question is from our CDL Practice Tests practice test. Take the full practice test to test your knowledge across all Shifting Gears and other topics.

Take the General Knowledge practice test →

About this topic: Shifting Gears

Most heavy commercial vehicles use non-synchronized manual transmissions, which require the double-clutch technique. Lugging or over-revving the engine wastes fuel and damages the powertrain. This is one of 4 General Knowledge questions on shifting gears, each with a full explanation on a single study page.

New to this exam? Our CDL exam guide explains the format, scoring, and how to prepare.

Related questions

Shifting Gears

When using a non-synchronized manual transmission, what is the proper method to upshift?

 Shifting Gears

When should you downshift a commercial vehicle?

 Shifting Gears

What is the proper technique for upshifting in a non-synchronized transmission?

State-specific guides

Need information for your state? Our state guides cover local requirements, fees, and what to expect on exam day.

California Texas Florida New York Pennsylvania Illinois Ohio Georgia

Recommended study materials

Study guide

CDL Study Guide (Trivium)

Complete prep for all CDL written tests: general knowledge, air brakes, combination vehicles, hazmat, and all endorsements. Includes full-length practice tests.

Check Price → Study guide

CDL Practice Tests & Flashcards (Accepted Inc.)

600+ practice questions covering every FMCSA knowledge area, plus 400 flashcards for endorsement prep.

Check Price → Study guide

Master the CDL Truck Driver's License (Arco)

Arco's classic CDL prep with in-depth FMCSA manual coverage, endorsement chapters, and scored practice exams.

Check Price →

As an Amazon Associate, CJ Affiliate, and AWIN publisher we earn from qualifying purchases. Links above are affiliate links. Full disclosure →