my last bus was a shortie blue bird with a 454 Chevy in it that I did some work on to make it nice N fresh.. cammed with an RV cam and it got 5-6 on average.. I dont know the exact transmission... 545 maybe? it was a 4 speed auto
I recently purchased a 92 ford b600 bluebird 52 passenger bus. It also has the 6.6 inline diesel in it. I get from 10- 15 mpg with it. Its also a 10 speed manual transmission so maybe that has something to do with it.