Aerodynamic drag — the resistance a vehicle faces from the air — grows exponentially with speed. Understanding this relationship helps you make fuel-efficient choices on longer journeys.
The Physics
Aerodynamic drag force is proportional to the square of speed. Fuel consumption follows a similar curve at highway speeds. The result: driving at 120 km/h instead of 90 km/h uses approximately 40–60% more fuel per kilometre depending on the vehicle. For a 500 km motorway journey, this can mean an extra 15–20 litres of fuel.
Optimal Speed
Most passenger cars are most fuel-efficient in the range of 70–90 km/h. Below about 60 km/h, the engine operates less efficiently (too high a gear) or at higher revs (too low a gear). Above 90 km/h, aerodynamic drag dominates and fuel use climbs steeply.
Other Speed-Related Factors
Roof racks, open windows, and heavy roof boxes dramatically increase drag — remove them when not in use. Tyre pressure also matters: under-inflated tyres increase rolling resistance, which worsens fuel consumption at all speeds.
Exam tip: The relationship between speed and fuel consumption is not linear — doubling speed more than doubles fuel use at highway speeds. The exam may ask which driving habit saves the most fuel: the answer is maintaining a steady moderate speed.