A squared-off BMW shell often cheats the air better than a soft, organic rival. That is not nostalgia; it is fluid mechanics enforced in sheet metal. Those old sedans carried drag coefficients near or below 0.30, not through sculpture, but through disciplined control of where air attaches, where it separates, and how the wake is shaped behind the car.
The key judgment from BMW engineers was brutal simplicity: stable flow beats pretty curvature. A crisp rear deck with a near-vertical cutoff lets the boundary layer detach cleanly, shrinking the low-pressure wake that drives drag, while many rounded tails force the air to cling, then shed vortices chaotically. Boxy C-pillars and defined edges act as deliberate separation lines, turning the whole body into a controlled diffuser rather than a random bluff object.
Another quiet advantage came from packaging restraint. Modest wheel arches, narrow tires, and uncluttered fascias reduced frontal area and interfered less with laminar flow along the sides. Underbodies, though simple, often formed a relatively flat plane, limiting turbulence under the floor. By contrast, modern styling demands wide stances, aggressive intakes, oversized wheels, and high ride heights, all of which inflate the product of drag coefficient and frontal area, even when the raw coefficient is competitive.
So the box won by saying no. No to swollen fenders, no to ornamental scoops, no to arbitrary creases that trip the boundary layer for the sake of drama. In those sedans, the wind saw fewer decisions made by marketing and more by the continuity equation.