At town speeds a vehicle spends most of its energy on rolling resistance and acceleration. At motorway speeds air resistance dominates, and it grows far faster than speed does.
The relationship is not linear
Drag force increases with the square of velocity, so a vehicle travelling half as fast again meets more than twice the resistance.
Power is force multiplied by speed, so the power required to overcome drag rises with the cube of velocity, which is the figure that matters for consumption.
This is why small increases in cruising speed produce disproportionate increases in fuel or energy use, and why the effect is barely noticeable in town.
Frontal area and shape contribute separately
Drag depends on the area the vehicle presents to the air and on the drag coefficient, which describes how cleanly air flows around that area.
A tall vehicle with a good coefficient can still consume heavily, because the area term is large regardless of how well the shape works.
Manufacturers quote the coefficient because it flatters, but the product of the two is the number that governs the vehicle's behaviour at speed.
Most of the drag is created at the rear
Air separating from the back of a vehicle leaves a turbulent low-pressure wake, and the pressure difference between front and rear is the dominant source of drag.
Tapering the rear so air stays attached longer reduces that wake, which is why efficient shapes have long, falling tails rather than only rounded noses.
Spoilers and roof extensions work by defining exactly where separation occurs, producing a smaller and more stable wake rather than preventing separation entirely.
The underside matters as much as the roof
Air passing beneath a vehicle meets suspension components, exhaust systems and structural members, all of which generate turbulence.
Flat underbody panels smooth this flow, and they also reduce lift, which improves stability as well as consumption.
Active shutters that close the radiator grille when cooling is not required address the air entering the engine bay, which follows a similarly disrupted path.
Wheels and arches are a further source, since a rotating wheel drags air around with it and the arch cavity generates turbulence of its own. Covering the wheel face and shaping the arch edge both help, which is why efficiency-focused designs often use wheels that look unusually closed.
Why electric vehicles pursue it hardest
Battery capacity is expensive and heavy, so reducing consumption at motorway speed is cheaper than adding cells to compensate for it.
Electric drivetrains also lack the engine bay cooling requirements that force large openings in the front of a combustion vehicle.
The result is that aerodynamic detailing has moved from a specialist concern to a visible influence on how mainstream vehicles are shaped.