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The German car magazine "Auto Bild" (2008-04-11 issue) tested the aerodynamics of cars in the Daimler wind tunnel.
www.autobild.de/mmg/mm_Bildergalerie_668619.html?tab=0&page=0
Drag factor = Cd*A, where Cd= drag coefficient, A= frontal area

Conclusions:

Already at 50 to 80 km/h (depending on the car), aerodynamic drag becomes the greatest loss factor
It's much easier to reduce drag than weight-related losses: reducing the Cd from 0.29 to 0.28 is like eliminating 100 kg vehicle weight
It's currently feasible to achieve a Cd of 0.22 in production cars, but with compromises in styling
You can't tell how aerodynamic a car is just by looking
Taller vehicles (e.g. vans) usually have higher consumption because of larger frontal area, but in the case of the VW Golf vs. Golf Plus vs. Touran this is partially compensated by a better drag coefficient.
Top-of-the-line model (wider tires, more grill airflow) has greater consumption by up to 0.3 L/100km (see Mercedes E500 vs. E200)
Longer cars often have better Cd, so that they have better aerodynamics than a small, short car despite larger A (see Peugeot 407 vs. 207)
Important details are: A pillars, underbody covering, lowering by 20 mm reduces Cd by 0.01, long front/rear overhangs are advantageous, rearview mirrors, separation area at the rear (small is better), small wheel wells (little air dams in front of the wheels help prevent air from entering them)
Convertible top down is really bad, folding hardtop is somewhat better than softtop (see Mazda MX-5)
Crossover cars (4x4) are worse than their standard counterparts (see Audi A6 Allroad vs. A6 Avant)
Flat sportscars are not always better (see Lamborghini Gallardo vs. Porsche 911 Carrera)
Carrying bicycles & ski boxes on the roof or on the rear can wreak havoc on the aerodynamics; the amount of worsening is greater on sedans than on station wagons (see Mercedes C 200 and C 200 T)
SUV aerodynamics suck (see BMW 530d Touring vs. X5 3.0d)
Station wagons are worse than sedans because the turbulent region at the rear is larger; so-called "lifestyle wagons" are not quite as bad, but have less cargo space (see Mercedes C 200 T vs. C 200)
Formula 1 racing cars have a terrible drag coefficient: 1.20
Mean drag coefficient has decreased over the years (1900: 0.95; 1920: 0.75; 1940: 0.58; 1960: 0.49; 1980: 0.42; 2000: 0.30). Some of the best: 1921 Rumpler Tropfenwagen 0.28; 1957 Citroen DS 0.37; 1966 NSU Ro 80 0.36
For current cars, the best Cd is 0.26 and the worst is 0.60 .

Here are the vehicle data: (Sorry, I don't know how to format the table here, so this is just a semicolon-delimited list. But you should be able to copy and paste this into a text editor, save it as a text file, and then open the text file with Excel, specifying ";" as the delimiter character. There must be a better way...)

Vehicle;Cd;A (m^2);Cd*A (m^2);increased consumption @ 120 km/h (L/100km);increased consumption @ 150 km/h (L/100km)
Taller vehicle:;;;;;
VW Golf;0.33;2.21;0.73;;
VW Golf Plus;0.32;2.38;0.76;0.1;0.2
VW Touran;0.31;2.55;0.79;0.2;0.3
Top-of -the-line:;;;;;
Mercedes E 200;0.27;2.22;0.60;;
Mercedes E 500;0.28;2.25;0.63;0.2;0.3
Small vs. midsized car:;;;;;
Peugeot 207;0.31;2.13;0.66;;
Peugeot 407;0.29;2.24;0.65;-0.1;
Convertible:;;;;;
Mazda MX-5 CC (folding hardtop);0.37;1.78;0.66;;
Mazda MX-5 Roadster (softtop);0.39;1.82;0.71;0.2;0.3
Mazda MX-5 Roadster (top down);0.45;1.80;0.81;0.7;1.4
Crossover:;;;;;
Audi A6 Avant;0.31;2.26;0.70;;
Audi A6 Allroad (normal setting 0.33, offroad 0.34);0.32;2.38;0.76;0.2;0.3
Sportscar:;;;;;
Porsche 911 Carrera;0.27;2.00;0.54;;
Lamborghini Gallardo (15 cm flatter than Porsche);0.33;1.91;0.63;;0.6
Bicycles & ski boxes (Mercedes C 200 & C 200 T):;;;;;
Sedan;0.27;2.22;0.60;;
Sedan, ski box on top;0.33;2.36;0.78;0.5;0.8
Sedan, bicycle on top;0.36;2.42;0.87;0.7;1.2
Sedan, bicycle on rear;0.37;2.22;0.82;0.8;1.4
Wagon;0.30;2.20;0.66;;
Wagon, ski box on top;0.32;2.41;0.77;0.2;0.3
Wagon, bicycle on top;0.37;2.43;0.90;0.6;1.0
Wagon, bicycle on rear;0.32;2.25;0.72;0.2;0.3
Wagon, ski box & bicycle on top + bicycle on rear;0.52;2.17;1.13;2.0;3,0
SUV:;;;;;
BMW 530d Touring (wagon);0.29;2.28;0.66;;
BMW X5 3.0d (SUV);0.33;2.88;0.95;;2.0
Station wagon:;;;;;
Mercedes C 200 (sedan);0.27;2.22;0.60;;
Mercedes C 200 T (wagon);0.30;2.20;0.66;;0.5
Ten best drag coefficient:;;;;;
BMW 5 series;0.26;;;;
Mercedes S class;0.26;;;;
Lexus LS 460;0.26;
Toyota Prius;0.26;
Audi A4;0.27;
BMW 3 series;0.27;
Mercedes E class;0.27;
Mercedes C class;0.27;
Mercedes CL;0.27;
Porsche 911;0.27;
Ten worst drag coefficient:;;
Formula 1 race car;1.20;
Mercedes Actros (aerodynamic cab-over-engine truck);0.81;
Morgan Roadster;0.60;
Land Rover Defender;0.59;
Hummer H2;0.53;
Mercedes G model;0.52;
Morgan Aero 8;0.51;
Jeep Wrangler;0.49;
Jeep Commander;0.41;
Chrysler PT Cruiser;0.38;