We have a 2000 3500 Chevy van with the 5.7 motor 200,000 miles.We get 14.5 to 15.5 fully loaded highway ( and an amazing 18 MPG @55 MPH). 12 or so around town.
The most efficient cargo vans will get about 20 miles per gallon on the highway and 15 miles per gallon for city driving. The most efficient cargo van is the Chevrolet Express.
Depends on the make, model, year, engine size, load you are carrying, and how you drive. Know that you will be lucky to get 17 mpg on such a vehicle. You will more than likely get around 13 in all...