What is the 2007 Chevrolet Silverado 2500HD Fuel Economy?

I bought a 2007 Chevrolet Silverado 2500HD, 6.6L Duramax Diesel, 4x4, Allison Transmission, 6-Speed Automatic, Crew Cab, 6 1/2 Ft Box. So I was wondering what kind of fuel economy I would get, you know mile to the gallon, and also how big is the fuel tank, so I know, in general, how much purple diesel to take with me on a decent length trip.