So Bozeman to Bakken Oil fields is >400 miles. there and back is >800 miles.
800 miles @ 75MPH is 10h 40min. 800miles @ 85MPH is 9h 24min.
Savings of 1 hour 15 minutes, if you managed to keep average speed close to speed limit, which surprisingly is not horribly hard around that area. I live in South Dakota, and there really is not a problem. Almost no traffic and fairly flat and straight roads.