Vehicles use the greatest amount of fuel when what action is performed?

• A. Idling
• B. Cruising at a constant speed
• C. Braking
• D. Accelerating

Answer: (D)

When you speed up, the engine has to work much harder to deliver greater power. That higher power demand means more fuel is burned to produce the needed energy. Accelerating not only increases your speed, but also fights against inertia, air resistance that rises with speed, and other losses in the drivetrain. The energy required to raise the car’s kinetic energy and to overcome these resistances is drawn from fuel, so fuel consumption spikes during acceleration.

In contrast, idling uses fuel at a relatively low rate just to keep the engine running while the car isn’t moving. Cruising at a constant speed keeps the engine at a steady, efficient level, so fuel use is lower than during rapid acceleration. Braking doesn’t require fuel to be burned; it dissipates the car’s kinetic energy as heat (and, in modern systems, some energy may be recovered, but that still isn’t the same as consuming fuel for propulsion). So the action that causes the greatest fuel use is accelerating.