The Physics of Fuel Consumption
A vehicle moving at constant speed must overcome two primary forces: aerodynamic drag and rolling resistance. Aerodynamic drag — the air pushing back against the vehicle — depends on speed squared, the drag coefficient (Cd), and the frontal area. Rolling resistance — the deformation of tires against the road — is roughly constant regardless of speed. At low speeds, rolling resistance dominates; at high speeds, drag takes over. The crossover typically occurs around 60–80 km/h, which is why this speed range is the sweet spot for fuel efficiency.
The Tyranny of the Cube Law
While drag force scales with v², the power needed to maintain speed scales with v³ — because power equals force times velocity. This cube relationship means that driving at 130 km/h requires roughly 3.7 times more power to overcome drag than driving at 80 km/h. Since engine fuel consumption is roughly proportional to power output, this explains the dramatic increase in fuel consumption above 100 km/h that every driver notices at the pump.
Drag Coefficient: The Shape of Efficiency
The drag coefficient (Cd) is a dimensionless number that captures how aerodynamically slippery a vehicle is. A flat plate perpendicular to airflow has Cd ≈ 1.0. A typical sedan achieves 0.25–0.35. The most aerodynamic production cars (Mercedes EQS, Tesla Model S) reach 0.20–0.22. Reducing Cd is achieved through smooth underbodies, tapered rear ends, flush door handles, and carefully designed mirrors. Every 0.01 reduction in Cd saves approximately 0.1–0.2 L/100km at highway speeds.
Optimizing Real-World Efficiency
The simulator models steady-state cruising, but real-world driving involves acceleration, deceleration, hills, and stops. Regenerative braking in hybrids and EVs recovers some kinetic energy lost during braking. Eco-driving techniques — gentle acceleration, anticipating stops, maintaining tire pressure, and removing roof racks — can reduce fuel consumption by 10–25 % without any vehicle modifications. The single most effective change most drivers can make is simply reducing highway speed from 130 to 100 km/h.