P.S. a well polished car should get higher mpg too, although this is perfectly safe the effect is minimal.
It would be nice to think so, and is certainly true of wings on aircraft, subsonically anyway.
However, the bulk of drag on a car is not skin friction, neither does it have a large wetted area c.f. its frontal area. This means it is effectively a blunt body, where pessure drag predominates.
As with a smooth golf ball, a boundary layer that is laminar cannot sustain such a large pressure recovery as flow decelerates again over the maximum section and thus has higher pressure drag...
So it's quite possible that enough imperfections (roughness) can create enough turbulence in the boundary layer to move the separation point back enough to easily outweigh any small additional skin friction by a reduction in overal pressure drag.
The best effect is to drive in hot summer air, which is thinner... or heat the bonnet, wings and roof
So a matt black car may well have less drag than a polished white one (in bright sunshine)
A hot golf ball also travels further, dimples or no dimples.