Aerodynamic drag

The aerodynamic drag force slows a body moving through a fluid (air, water):

$F=\tfrac12\,C_d\,\rho\,A\,v^2$

where $C_d$ is the drag coefficient (shape-dependent), $\rho$ is the density of the medium (air ≈ 1.29 kg/m³), $A$ is the cross-sectional area, and $v$ is the speed.

The key feature is the dependence on the square of the speed: driving twice as fast means four times the drag (and sharply rising fuel use). A streamlined shape (low $C_d$) drastically reduces drag — a droplet has $C_d\approx0.04$, whereas a flat plate reaches $\approx1.3$.