Thanks ,Guys-Kevin
http://topdefinitions.com/2013/07/28/drag-coefficient-is/
A couple of clarifying points. First, cd varies with speed and secondly, as illustrated by the chart on the right when you look a the cross section of comparable cylinders, one short, one long, you see a significant difference in cd.
But, as kayak builders will tell you, at lower speeds in a fluid medium, two simarly shaped objects, one short, one long, will not have as great a difference in cd. Because the longer object has a higher speed potential, the difference in cd is great at higher speeds. That is why below 45 mph, two simarly shaped cars, one short, one long, could have the same mpg rating. As the speed increases, the difference gets greater. But, as you read the article, there are a lot of variables.
Most importantly, the lowest cd on a streamline shape shape ( less form drag) illustrated in the chart may be good for a foil, but it makes for a vary poor shape for a car that’s short because of lower roof line. So, the other advantage of longer ( and larger) cars, is that this shape can be maintained while still providing an adaquate amount of interior space. You look at a longer Accord and you can see why, cvt not withstanding, this heavier, roomier, longer, more powerful car can get highway mileage that rivals or exceeds subcompacts.
So, it’s not rocket science ( pun). It’s why displacement boats with long overhangs sail fast and more efficiently healed over and why Corollas get so much better highway mileage then " dumb" ( not so)Smartcars.
While low Cd is a very important parameter for good mileage, I’d argue that there are other very huge contributors.
Witness the Veyron, which has a very low Cd but needs special fuel pumps and large fuel lines just to keep it fed. It’s been written that at full throttle it’ll drain its 20+/- gallon fuel tank in 10 minutes flat. Or any o of countless other low-Cd supercars, like the Pagani.
At the other extreme is the new Nissan Delta Wing that they’ll be racing at the next LeMans. Its Cd is so low that they’ve calculated that they can run an extra lap between fill ups at racing speeds just on the electric motors.
Perhaps it would be more accurate to suggest that a vehicle’s Cd determines the least possible air resistance that it has to overcome.
@dagosa: Cd varies with speed (reallly Reynold’s number) somewhat, but technically speaking, if it varies, it’s not a coefficient.
Drag varies with speed. The “C” in “Cd” literally means “coefficient”.
" if it varies, it’s not a coefficient"
You know, we could avoid all these little technical, so called corrections if we just read the reference. I suppose I could have just referred to it and not paraphrased the article. Sometimes, saying something in gereralities without trying to get too technical is harder then teaching math to kids for 37 years. @meanjoe75fan Cd literally means drag coefficient. Once figured as a variable dependent upon other factors, like speed, it then becomes constant for all other computations when comparing more then one shape under the same conditions. Don’t take the word coefficient literally . Otherwise, we would call it a constant. The 9.8 in 9.8m/second squared is a coefficient for acceleration due to gravity on earth. But it is variable depending upon the planet. Coefficient just means, the number that goes with another number as a factor. It does not mean it is always constant !
So, saying a coefficient can not be a variable too…is incorrect or correct, depending upon the context. Once you find it, it’s a constant. Before you find it, it’s a variable !!
That’s what you get for trying to distill a complex topic down! I didn’t realize all the complexities associated with Cd:
"The drag equation is essentially a statement that the drag force on any object is proportional to the density of the fluid and proportional to the square of the relative speed between the object and the fluid.
Cd is not a constant but varies as a function of speed, flow direction, object position, object size, fluid density and fluid viscosity. Speed, kinematic viscosity and a characteristic length scale of the object are incorporated into a dimensionless quantity called the Reynolds number or Re. Cd is thus a function of Re. In compressible flow, the speed of sound is relevant and Cd is also a function of Mach number Ma .
For a certain body shape the drag coefficient Cd only depends on the Reynolds number, Mach number, and the direction of the flow. For low Mach number, the drag coefficient is independent of Mach number. Also the variation with Reynolds number within a practical range of interest is usually small, while for cars at highway speed and aircraft at cruising speed the incoming flow direction is as well more-or-less the same. So the drag coefficient Cd can often be treated as a constant."
Exactly @texases. We use it as a constant when we want to make meaningful but general comparisons between different objects. Like cars.
Coefficients can vary for a lot of reasons. But, as Dag said, we use it like a fixed number for comparisons. The standard used is probably regular ambient conditions, 29.92 in. hg., 23C, 50% humidity. Compensation curves are probably used to adjust for nonstandard conditions. I’m just guessin’, you understand
My intended point was that the claim that “C” isn’t a coefficient is sort of self-defeating, since “C” literally means coefficient.
Maybe a Smart could be stretched out and area ruled… 
Hmmm,apples and oranges ,at highway speed a Mack truck will get 6-7 mpg,grossing around 27 tons and its Cd is the other side of horrible an old Air force guy told me one time that drag on a standard Jet Fighter of His day was probably something akin to a 4x8 sheet of plywood being pushed through the air at very speeds flat ways,so I guess there are sweet spots in everything.It seems that diesel truck running at highway speeds with its horrendous weight and terrible drag must actually be more efficient then a compact car that weighs less then a ton and a half and has a much better Cd.I guess its all relevant,like a Mennonite Machinist once told me when we were discussing the the merits of Hydraulic drive on his small “Stiener” tractor-not terribly efficient,but in that case good enough.So I suppose it all boils down to what works well enough on the average,the law of diminishing returns will bite us sometimes,just as the emission systems on cars,when is enough,enough?when the low hanging fruit is gone,maybe its time to dig the potatoes-Kevin
@thesamemountainbike
I see what you are saying and would like to expand on it, just because I have retire and have no one else to talk to while the Celtics took a time out.
I apologize for getting technical but the drag coefficient is (Cd) It is not C times small d where C is the coefficient. Just calling a number C in physics a coefficient and left undefined is not enough. Technically, when using the coefficient of drag as a coefficient in a numerical phrase, with the variable “x” for example, you must say (Cd) x . Other wise , just calling it a coefficient © makes it indistinquisable from the coefficient of friction and others like COR ( coefficient of restitution). It’s just a technicality. But C is not a constant for d. Cd is a variable if unknown ( a variable is just an unknown number) and a constant if known. In the product x times y or xy, x is the coefficient for y even though both are variables. Coefficients can be known or unknown, constants or variables. That’s the point I was trying to make earlier when “mean” said coefficients can’t vary…
So technically, I would say Cd is the drag coefficient, no more no less. The reason being, any number can be a coefficient of another and it doesnot need the letter C to define it that way.
Question…if a=1 and z= 26 then the letters in the product of ( samemountainbike) = ?
Well, maybe the drag of the Smart could be reduced if it was area ruled…
It might put a crimp in those seats though… 
As others noted, the Cd values we see are probably taken at a standard velocity, temp, pressure, etc. When that’s the case then it is a constant, for comparative purposes.
It’s a constant for one shape, but it varies from one shape to another. So have to be real careful calling anything a constant. The Cd for Smartcars maybe 1.4 ( made up), and that’s a constant. But, when comparing cars, Cd is technically a variable. So, I would just add to your statement, it’s a constant “for each shape (car).”
Now I’ll disagree. For comparing cars a standard conditions Cd measurement is quite comparable. How would you better compare them?
@texases …l.lreally no need to disagree. I am only speaking technically with your use of the word “constant” Why ? Because I’m being a PITA. See, I said that each Cd for each car IS constant. But when you campare cars, you must vary the Cd to make the comparison. So technically, nothing is ever constant unless strictly defined. So all I suggest you do is, say Cd is a constant “for each shape” when you compare them.
Remembering that your Cd will have to vary while making the comparison, otherwise, all cars would be the same ! It does not conflict with your basic premise at all. This is the crap I put my kids at home through…and my wife was an English teacher…yuck.
Today I saw a Smart Car in a Target parking lot. I noticed on its license plate that it was from a county at least 200 miles away. The interstate between my county and this one is full of big rigs. I can’t imagine driving one of those “Quasimotos” on the interstate!
Stop thinking of coefficients as constants. They are meaningful values that can be compared to pick the one most satisfactory for a particular problem.
One more thing: length on the water line is not enough to define maximum speed. It is the ratio of length to width that matters. That is why a 40’ catamaran has a higher top speed than a comparable monohull boat.
Trust me dag, I understand. I also recognize that C is often referred to as if it were a constant multiplied by a variable, which it is not. It is “C of d”, not “C times d”. The coefficient of drag is a multiplier for a variable, not two separate things.
It was Joe’s comment that “Cd varies with speed (reallly Reynold’s number) somewhat, but technically speaking, if it varies, it’s not a coefficient.” to which I was responding. In this case “C” is used to designate the coefficient, and is therefore, by definition, a coefficient, which made Joe’s comment sort of contrary to the very definition.