Would dimples on the surface of a car reduce the surface resistance and produce greater fuel milage? (Simular to how a golf ball works)
Even if it did, there are other considerations, it would be expensive in manufacture, to guaruntee the uniformity and quality of the finish, and somehow I doubt it would look all that good, and none of these would justify any reduced drag, in fact its highly likely that the drag wouldn’t e reduced by much at all.
The killer though is that the panels also carry mechanical loads, they are structural, and the dimple surface would probably not be as strong, you are also increasing the surface area, more paint, more material in the panel and more weight.
I know what you mean. I have wondered for a couple of years if Major League Baseball would allow a bat with dozens of team logos burned into the barrel of the bat along with the bat company’s trademark. It seems to me it would help bat speed.
IIRC, dimples are better for aerodynamics of relatively small things that winds goes around, not just over. They reduce the drag from the wake behind an object. Some bicyle shirts have dimpling on the arms but are smooth on the chest and back. Dimpling might be effective on the external mirrors.
Folks have been talking about dimpling cars at least since the 80’s. It never seems to amount to anything.
Here’s a recent iteration:
Dimpled Aerodynamic Surfaces
The difference would probably be too small at the speeds we drive.
Racing type .yachts have experimented with shark skin type dimpling for a while though. I think they still do it.
It would be difficult to mass produce a car surface like that.
Oh, rats! Now I’ve got the start of a blues song running through my head.
“I got dimples on my balls, baby,
My laminar flow is mighty good.”
Here’s an example of of a relatively mass-produced dimpled carbon-fiber bicycle wheel.
So you definitely put them into composites, but I’m not certain if they would be effective at automotive speeds, but they definitely seem effective at 30-35 MPH and more.
Body panels on unibody cars almost never serve any structural purpose, although they may contribute to passenger injury prevention during a collision event. In many cases, dimpling and other types of character lines are added to augment the rigidity of body and structural sheet metal. You’ll see, for instance, that there’s no truly flat surface anywhere on a sheet metal body; there’s always some minor degree of radius. These add no appreciable cost to panel production; it’s just part of the stamping die.
As to dimpling working for wind resistance, consider that the front structure of the car and the forward roof bow are the primary resistances to wind. In a well-designed automobile, there will very little wind on the flat surfaces that you would want to dimple; the wind is properly diverted at the primary leading edges.
Speed holes, though, do work. 
You wouldn’t want to put dimples on cars for the simple reason that you use dimples on golf balls only because you never know which side is facing forward. When you know which side is forward, there are much better ways to delay flow separation and recirculation, the thing that the dimples are used for.
The reason the dimples work is that the golf ball is a sphere. As the flow passes over it the ball surface “drops away” from it on the back half. This means that for the flow to travel close along the surface of the ball it would have to change direction very quickly, which it does not like to do. Instead, it separates. This means that a flow line (the line made by putting smoke into the air passing over the ball, like you see in car commercials) that was on the surface will stop following it, instead traveling more straight back past the ball. The air past the separation point is recirculating. Vortexes of air make circular patterns that stay near the back of the ball instead of flowing downstream. This creates a huge amount of drag.
When you have laminar flow (nice smooth flow lines, like the smoke just after it comes off a blown out candle wick) it has little drag as it passes along the surface of the ball, but it separates very easily. When you have turbulent flow (all mixed up flow lines, like you have a few inches above the wick of a blown out candle) it has more drag as it moves along the surface of the ball, but it stays attached to the ball surface much better, and delays separation. You want turbulent flow because you save a lot more drag in having less recirculating air than you lose by having more drag while the flow is attached.
So, turbulent is good and laminar is bad (for this case). Thus, they came up with those little dimples. They mess up the flow going over the ball, turning it from laminar to turbulent and thus reducing the drag.
Ideally, we could put some kind of fairing on the back side of the ball that would make it so the surface doesn’t drop away from the ball so quickly, eliminate recirculation entirely, and have huge savings on drag. If you were to do that you would end up with a cross-section that looked suspiciously like an airfoil. As the ball is spinning and the “front” is always changing (not to mention that it would be very hard to hit the ball on the back pointy end) it is not feasible to add a fairing.
In a car, though, you don’t have to worry about smacking it with a golf club, and the same side is facing forward for all but very low speeds, thus we have come up with the nicer, smoother shapes you see on modern vehicles. Adding dimples wouldn’t slow separation (for most cars I think the only major separation occurs at the squared off back, and there is no way you are going to delay that) and would add drag of its own, so they are not used.
Basically, gold balls have a different kind of drag problem than cars and the solution for golf ball drag wouldn’t do anything about car drag.
You could shoot a put of “speed holes” in the car with a pickax to make it go faster. Worked for Homer Simpson, anyway.
My references are always stolen before I can make them. 
This would certainly have been discovered by someone in the midwest. I once rented a new car for a weekend, and was caught in an Iowa hailstorm. That Pontiac looked like Lionel Hampton had been playing the hood, roof, and trunklid with ball peen hammers. It didn’t seem to go any faster on the way back to drop it off.
How big do the dimples need to be? Just wondering as modern urethane auto paints are not glass-smooth like the old lacquers or enamels were. The stuff goes down with a slightly bumpy/dimply finish, so most cars already have some level of dimpling.
Many people have dimples on their faces, but I haven’t noticed that the dimpled people are the fastest runners. Maybe we should have sprinters press waffle irons to their faces before each event.
On the other hand, I think this post crossed the silly line about eight strides back. :smack:
It is all a matter of how quickly you transition to turbulent/how much drag you pay to do it. Sailplanes are probably the vehicle type most concerned about drag in the world. They keep everything super smooth and then add a very narrow strip of sandpaper-like grit exactly where they want the flow to transition. If you wanted to do anything to the surface of the car you would do this. Things like the edges of windshields and sunroofs probably already do it for you though.