I know a guy pulling a reefer with a 386. He is an O/O and came out of a 379. He has a ISX 525 Cummins and says he gets about 6.5MPG. He also says he would rather have his "hood" back.
I own a 386. It has a Cat 475 10 speed. I pull flatbed and usually grossing 75,000 plus. I'm averaging around 5.7 mpg. The turning radius on the truck is great, rides ok, interior room is on the tight side, especially between the seats.