A turbine-shaped wheelcover to go with Lego Technic Racing Medium wheels (56x34mm). This is the right-hand version. The left-hand version is available too. The old version of this part has been printed on a different printer, albeit slightly inaccurately.
This new version only changes the axle size for a better fit.
Of course left and right are only suggestions--they can be swapped for a different look. ;)