It is probably too complicated to be sure, with subtle shifts in
air flow easily affecting the total drag. However, since things
(usually) tend towards their lowest energy, and the wheels tend
to spin when the car is moving, I would guess that is the lowest
energy state, and hence the lowest total drag, compared to
clamping the brakes on the bike wheel and stopping it from
spinning. But that statement, lowest energy state, and hence the
lowest total drag, is BS. Things have no tendency towards a
state of lowest energy dissipation! Drag on the car * speed =
power from the car. power from the car = power from the spinning
wheel + everything else Increased drag on the car = power the
spinning wheel generates / speed If the wheel does not spin, it
does no work. If the wheel spins freely, it does -very little
work-, because it has very little friction. So the spinning
wheel adds very little more drag than the fixed one.