1997 Chevy Silverado 2500. I pulled out the stock 350 motor and dropped in a Chevrolet 383 crate motor. I get 8-10 mpg with the camper. That *maybe* dropped 1 mpg from the stock 350. That may not be a variable related to the engine displacement, that is most likely related to the additional power available under my right foot. It now climbs hills at the posted speeds, rather than screaming 2nd gear & 45mph.
I suppose this seems like an older truck at 22 years old.... It lives in a heated shop, so it is by no means an old beater. Heading out for a camping trip in a few weeks. Went to check and see if it is due for service...and realized I have only put 400 miles on the truck since last August.
Unloaded is around 14 mpg. Extended cab, full bed, 4x4 chassis...is a lot of weight to haul around. I do tend to 'exercise' the 383 when running light, so again that number is probably mostly related to my right foot. :)
-Eric