Aerodynamic drag is not proportional to speed. It is proportional to the square of it, and the power needed to push through it is proportional to the cube.
drag = ½ × air density × speed² × frontal area × drag coefficient
A car-sized object with 1.5 square metres of frontal area and a drag coefficient of 0.3, at 25 metres a second, meets 172.27 newtons of drag and needs 4,306.6 watts to maintain it. The aerodynamic drag calculator reports both, because the force is what you feel and the power is what you pay for.
The cube is the expensive part
Halve the speed to 12.5 metres a second, about 45 km/h, and the drag falls to 43.07 newtons and the power to 538.3 watts. Double it instead, to 50 metres a second or 180 km/h, and the drag is 689.06 newtons and the power 34,453.1 watts.
Those three power figures are 538, 4,307 and 34,453. Each doubling of speed multiplies the power by eight. That is the cube, and it is the reason a vehicle that cruises comfortably at one speed is working extraordinarily hard at twice it, while the speedometer suggests a modest change.
It also explains why the top speed of a vehicle is such a poor guide to its engine. Going fifteen per cent faster at the top end needs roughly fifty per cent more power, so the last increment of speed consumes an enormous share of the capability.
Why the saving is never what you expect
Drag is not the only resistance. Rolling resistance is roughly constant with speed, and the engine has its own losses, so total consumption does not follow a pure cube. But drag dominates above about 70 or 80 km/h for most vehicles, which is why fuel economy falls away so sharply on a motorway and why a roof box, which adds frontal area and spoils the coefficient, costs so much more at speed than it does around town.
The two parameters you can actually change are in the formula. Frontal area enters linearly, so halving it halves the drag exactly, as the calculator shows. The drag coefficient also enters linearly, and it is the one that rewards shape rather than size: a tenth off the coefficient is worth as much as a tenth off the area.
The same equation sets terminal velocity
Falling objects speed up until drag balances weight, and then stop accelerating. For an 80 kilogram skydiver presenting 0.7 square metres with a drag coefficient of 1, the terminal velocity calculator gives 42.78 metres a second, which is 154 km/h, reached with 784.5 newtons of drag exactly balancing the weight.
Change the posture and the whole figure changes, because area and coefficient are what the skydiver controls. That is the same lever as the roof box, used deliberately.
For the time side of speed rather than the energy side, there is the post on why speeding saves less time than you think, and the mpg illusion covers why fuel economy figures mislead even before aerodynamics get involved. The drag coefficient calculator works the other way, from a measured force back to the coefficient.