Why does motorway speed cost so much more fuel?
Because aerodynamic drag rises with the square of speed, and the power needed to overcome it rises with the cube — so going faster costs far more than the speed difference suggests.
The arithmetic. Drag force is proportional to velocity squared. Power is force times velocity, so power required to push through the air scales with velocity cubed. Going from 60 to 75 mph — a 25% increase in speed — requires roughly double the power to overcome drag.
Why this dominates at motorway speed. At low speeds, rolling resistance from tyres is the main opposing force and is roughly constant. Aerodynamic drag is small. As speed rises, drag grows rapidly, and somewhere around 50–60 mph it overtakes rolling resistance and then dominates everything. Below that, drag barely matters; above it, drag is the story.
The two numbers that determine drag:
Drag coefficient (Cd) — how efficiently the shape moves through air. Modern cars achieve remarkably low figures; electric cars are pushed lower still, because range is the headline number.
Frontal area (A) — how big a hole the car punches in the air.
It is the product, CdA, that matters. A very slippery but tall SUV can have worse real drag than a boxier but smaller car, which is why quoting Cd alone is misleading.
Why electric cars are affected more. Range anxiety makes the loss more visible, and EVs are more efficient at low speed — they lose nothing idling, and recover energy braking — so motorway driving is where their efficiency advantage is smallest. An EV's range at 70 mph can be substantially below its rated figure, while a diesel's motorway economy is frequently its best.
What else costs you at speed:
Roof boxes and roof bars — among the largest penalties available, and frequently left on for months unused. Bars alone cost several percent.
Open windows at speed, worse than air conditioning above roughly 45–55 mph.
Towing, which adds both frontal area and turbulence.
Underinflated tyres, raising rolling resistance.
The practical point: reducing motorway speed modestly produces a disproportionate saving, and removing roof bars is free.