In the quest for ever more efficient and aerodynamic cars, automatic shutters have made their appearance in car grilles. Applying the same to wheels is new, where shutters ensure the surface area of the wheel becomes flush at higher speeds. This and several other measures combined can lead up to an improvement of 2 MPG on a highway.
Ford Atlas Truck Concept Demos Fuel-Saving Active Wheel Shutters
These wheel shutters align with the spokes of the truck’s alloy wheels when at rest or at low speeds, yet at highway speeds they automatically close so as to create a nearly flush wheel surface. Energy harnessed from the motion of the wheels (recharging batteries) would provide the means of opening and closing the shutters, while they’d also open during heavy-duty use like towing, or rapid braking.
This also reminds me of what is currently installed in the Model S, retractable door handles: