It All Depends...
Judging accurately how many miles you have left in the tank after that little yellow 'low fuel' light illuminates depends on one thing; city or highway!! If you're running straight down the PA Turnpike at a cruise-controlled 70 mph you can probably expect to make the rest stop that's still thirty-five miles east of you. If you're stuck in King of Prussia in the middle of the afternoon you better have a destination within fifteen to twenty miles in mind. Why? Fuel mileage drops by almost half when you take your Lux-O-Barge off the highway and start the stop-and-go shuffle at thirty-some miles an hour.
The most fuel efficient speed is around 50 mph (approximately 80KM/H) I agree with the driving method though. Steady speed with minimum braking and acceleration. I did this on a remote gravel road; however, it ruined my suspension.