My 79 300SD seems to like slightly higher speeds...around 70 or 80 mph. It's very smooth in that range, and it doesn't feel that fast. This could be bad for my driving record!
Whenever I'm on a road-trip or just on the open highway for any length of time, it just seems very comfortable at such speeds, and wants to "creep up" there if I'm not paying attention to the speedo. Don't know about the difference in fuel mileage...never calculated it that closely.
Mike