1) in fluid mechanics there's a concept of 'dynamic similarity.'
2) the giant world globe of the past New York's World's Fair, a Charters -Thomas cannon ball from a Civil War artillery piece, and a B-B from a B-B gun would have identical drag coefficients, as long as Reynolds number criteria were respected.
3) below a critical Reynolds ( Rn ) number of around 500,000, all spheres have a Cd 0.47.
4) if these spheres are accelerated beyond a velocity capable of producing an Rn above 500,000, by around Rn=1,000,000, their drag drops to Cd 0.10.
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5) 'artificial roughening', via gluing sand to the sphere, or 'dimpling', amplifies the Rn for a given velocity, accelerating the transition from a laminar boundary-layer ( LBL ), to a full turbulent boundary layer ( TBL ).
6) the vast momentum interchange between the local streamline just above a TBL and the surface of the sphere's body enables the sphere's flow to remain attached within the hostile positive ( adverse ) pressure gradient present in the sphere's aft-body, without which, all the flow would attempt to flow 'upstream' towards the highest suction peak, just ahead of the 'equator' of the sphere.
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7) for the 'length' of commercially-produced road vehicles, at, and above around 20-miles per hour, Hucho attributes automobiles with Rn 10,000,000, and they all have this super-critical Rn which ensures a full TBL, capable of maintaining fully-attached laminar aft-body flow; as long as the 'shape', 'form,' 'contour', 'silhouette', of the aft-body is 'streamlined.'
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8) consequently, if we were 'Lilliputians,' driving 'Hot Wheels' or 'Matchbox'-sized vehicles, ' dimples' would be of help, helping facilitate the development of the TBL required to allow fully-attached laminar flow outside the boundary-layer of the tiny cars, as mandated under the conditions of verisimilitude.
9) Short of that, they're essentially 'useless.'
