Drag coefficient

[Diesel_Dave]

Yes, technically, Cd does depend on Reynold's number, surface roughness, etc, etc., but those effects are pretty small. It's a fairly common assumtion that Cd is constant for a given shape, however. That's why there can be a Cd quoted for a given car. Technically a car will have different Cd's at different Reynold's numbers (and other factors), but often just assumed to be constant because those effects are fairly small.

[gone-ot]

...except for the Mythbuster idjiots, nobody I know of has "dimpled" their car's body-panels in an effort to reduce its Reynolds Number!

[GRU]

This is why i thind Cd is pointless advertising about fuel economy....Take any car and make it twice the size without increasing the weight... The Cd will be the same yet the original (smaller car) will have less wind resistance and get better fuel economy

[NeilBlanchard]

If you are comparing two very similarly sized cars, then the one with a lower Cd is likely to get better mileage. More information is better. And lowering the Cd is the best way to improve the efficiency, after improving the drivetrain.

[Tango Charlie]

Advertising a car's Cd is an indication that they've at least paid attention to it. Nothing but good can come of manufacturers producing more aerodynamic vehicles, bragging about it, and thus making the public aware of its impact on efficiency.

[aerohead]

Originally Posted by drmiller100

what you wrote makes no sense to me.

I don't compare fish to birds. I don't compare tractor trailers to hatchbacks.

Comparing Cd is worthless between the two.

Cda is what matters, because if you combine speed, Cda, and weight, you can KNOW the drag of the car.

Cd is a way to sort of calculate Cda, but it is kind of worthless by itself.

The Cd of a car would give a potential consumer the same information as the thermal R-Factor for doors,windows,walls,and ceiling in a prospective home purchase,as the inverse of R-Factor is the Heat Transfer Coefficient for the home.A thermal Cd if you will.
The square footage of the home wood give an overall sense of energy efficiency as would the frontal area in a vehicle.
It is quite proper to compare Cds of aircraft,birds,submarines,bullets,fish,radomes,'n such,as they are all subject to the same forces and we can learn from their performance vs shape.