GMC 3500 Value Van fuel question

[elbowroom]

Hi, I have the above van and I am looking to find out what the fuel capicty is and the average miles per gallon. Does anyone out there know this information or where I can find it?
Thank you.

[JoeyD]

What year? 6.2, 6.5, 6.6?

I have an 06 D Max van and with roof racks and loaded with my tools and stock I get anywhere from 17-19 most of the time. I do about 50/50 city highway miles. Stay on the highway and you will see better than 21