As it sits now, and with cold temps, approx 22mpg. @60mph.
This engine was in a Potato Chip Step van. Even a lot of the 70`s step vans that were built on GM chassis were converted to the 4BT (originally 6cyl. gas). The same engine is in 1845C Case skid loaders & some Case backhoes. I have seen telehandlers with a 4B in them.
I have a complete `77 GM chassis step van with a 4BT in it that was built with a straight six gas. eng. and converted about ten years later.