My '89 b250 will get 9 to 11 city, and 14.5 to 17.5 highway averaging low 16's at no more than 68mph.

318 TBI. After 93 or '4 they went with multiport fuel injection. these engines have higher power ratings but I have not heard of them getting much better mileage than I experience. Doesn't mean they can't though.

I carry a bunch of extra weight in tools, and other gear and have a raised fiberglass roof.