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There should be an acronym for "Teach your mother to suck eggs" . . . . I was iterating on CFD models of World Superbike motorcycles and testing vehicles in a wind tunnel when the Tesla Model S was still a gleam in Van Holthousen's eye. Yes, Cd, is a dimensionless number. So is the fineness ratio, which is worse for smaller cars. Also, the passenger space doesn't change all that much from smaller to bigger sedans, only increasing in length and perhaps height and width slightly. So a tiny car tends to be a block defined by the passenger space, without much room to streamline the nose and tail. Look at the very aerodynamic Mercedes EQXX, and notice how much the tail extends beyond the rear seating area. You can't do that on a 4-seat 4.2m car.
But as a practical aerodynamicist, I actually dislike talking about Cd's as they're often used to mislead. A good Cd on a Hummer stills means very large drag. The important number is the effective front area, CdA, which is what needs to be minimized. On a car that has a lot of length, you can do that with a carefully curvaceous nose that doesn't push the air much outward of the vehicle's maximum frontal area, and by converging the tail surfaces gently over a long distance, and thus achieve a low CdA by lowering Cd while A may remain large. On a small car, it's much more of a balancing act, requiring a decent Cd and a smaller A to get a similar or slightly better CdA result.
. 50 miles/kWh beats even our Tesla handily for efficiency.
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