IMHO that Mythbuster car dimple thing was about the looniest thing I've ever seen. It might make a great Golf Add Car though.
First, golf ball dimples work because at the "Reynolds Number" a golf ball lives at the air is effectively much thicker than say said automobile and builds up a huge boundary layer (compared to it's overall size) and the dimples act like mini tabulators and scrub the thick boundary layer reducing the "apparent" form size of the ball. As a result the air flow boundary layer is reduced allowing the air to more closely conform to the ball. The turbulent surface drag results in being less than the former apparent form drag. You can get the same effect by placing a small wire ring about 25% of the way back from the leading edge to trip the flow. Older Lear jets used that technique just ahead of the ailerons to re-energize and attach the flow from the thick boundary layer.
I think a Ball Peen hammer would work just as well. Or park your car outside in Denver during summer hail season. Or several seasons.
Seriously, the more the air has to bend and go around things the more energy that is imparted to the vehicle from the flow. This results in drag. IRL a car is a universe of high and low Reynolds numbers. For us mortals the nuances of it all are superfluous.
