Although speed is important for fuel efficiency, consumption is not determined solely by the speedometer. As speed increases, aerodynamic drag grows; the engine transmits more power to the wheels to maintain a constant speed. For this reason, there may be a difference in fuel consumption between 90 km/h and higher speeds on the highway. However, it is not a case of “the slower, the more economical.” At very low speeds, using the wrong gear, high load at low revs, or a gear that is lower than necessary can increase consumption. Even when speed is low in stop-and-go traffic, frequent acceleration, braking, and idling while waiting increase fuel consumption.
- Engine speed, gear, acceleration style, traffic, and road gradient affect consumption.
- SUVs and commercial vehicles may be more affected by aerodynamic drag because of their large frontal areas.
- Roof boxes, roof racks, and headwinds can increase consumption.
- A constant speed alone does not guarantee low consumption.
The aim of economical driving is to operate the vehicle smoothly in the appropriate gear and within the engine’s comfortable rev range. In a manual vehicle, shifting up too early can strain the engine; an automatic transmission manages this balance to a large extent. The ideal rev range varies depending on whether the engine is diesel or gasoline-powered, as well as on the turbo, engine displacement, transmission ratios, and load.
Why it matters
This framework broadens the criteria for drivers who assess fuel economy solely by looking at the speedometer. Those driving SUVs and commercial vehicles, as well as those carrying roof racks, should take into account that aerodynamic drag and load conditions can change the outcome even at the same speed. The fact that a low average speed in urban traffic does not automatically mean low consumption also requires considering traffic flow and frequent acceleration and braking. In manual vehicles, gear selection is the driver’s direct responsibility, while an automatic transmission manages this balance to a large extent; however, the fact that the appropriate rev range varies from vehicle to vehicle rules out recommending a single economical speed or gear. The question that remains is how to determine the range in which the engine operates comfortably under specific vehicle, load, and road conditions.