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Yahoo! compiled a list of the top gas guzzlers. Here's the list:

Cars:

1. Volkswagen Phaeton 6.0L 12cyl 5A 12 19
2. Mercedes-Benz S Class 5.5L 12cyl Turbo 5A 12 19
3. Mercedes-Benz CL Class 5.5L 12cyl Turbo 5A 13 19
4. Dodge Viper 8.3L 10cyl 6M 12 20
5. Ford GT 5.4L 8cyl S/C 6M 13 21
6. Audi S4 quattro 4.2L 8cyl 6M 15 21
7. Porsche 911 Turbo S 3.6L 6cyl Turbo 6M 15 22
8. Cadillac CTS-V 5.7L 8cyl 6M 15 23
9. BMW 6 Series 4.4L 8cyl 6M 15 23
10. BMW M3 3.2L 6cyl 6M 16 23

Trucks:

1. Dodge Ram Pickup 1500 SRT-10 8.3L 10cyl 4A 9 12
2. Mercedes-Benz G Class 5.0L 8cyl 5A 13 14
3. Land Rover Range Rover 4.4L 8cyl 5A 12 16
4. Chevrolet Silverado 1500 5.3L 8cyl 4A 14 16
5.Ford Econoline Wagon 5.4L 8cyl 4A 13 17
6. GMC Savana 5.3L 8cyl 4A 13 17
7. Lexus LX 470 4.7L 8cyl 5A 13 17
8. Cadillac Escalade 6.0L 8cyl 4A 13 17
9. Toyota Land Cruiser 4.7L 8cyl 5A 13 17
10. Dodge Ram Pickup 1500 4.7L 8cyl 6M 14 18

You can read the rest of the Yahoo! article here. It also outlines some other stuff too.  

Notice the Hummer H2 is missing because it's so heavy it's exempt from listing mileage figures.