Sorry, I didn't see your comment until now. The highway speed depends greatly upon the rear end gearing: mine will go 65 MPH (around 62 MPH seems to be its happy place), and the one I owned previously had plenty more to give at 78 MPH. I average 10-11 MPG with a turbocharged 671 Detroit Diesel and the same with my older, non-turbo 671 Detroit Diesels. I run regular highway diesel. Two-stroke Detroit Diesels should only be run with straight 40-weight oil in them, too. Don't use a multi-grade 15W40. Delo 100 or Shell Rotella 40W is what I have in all of mine. Maintenance varies; it's a machine. I haven't had to do much more than typical PM-type maintenance to any of mine, but it's important to have someone knowledgeable in these engines nearby if you, yourself, aren't already.