Next time you're near a late model pickup, take a peek inside its wheel wells. And compare that to late model passenger cars.
This wheel-house 'volume' reduction vs Cd, is something shown to reduce drag, in both wind tunnel and CFD investigations.
Since, like a belly pan, they don't 'show', designers have embraced them to gain efficiency, without risk to sales, and aerodynamic engineers have license to dial in 'optimums', allowing just enough for 'wheel flop' and snow chain install / removal clearance.
