http://eartheasy.com/move_fuel_efficient_driving.html
Drive steadily at posted speed limits.
Increasing your highway cruising speed from 55mph (90km/h) to 75mph (120km/h) can raise fuel consumption as much as 20%. You can improve your gas mileage 10 - 15% by driving at 55mph rather than 65mph (104km/h).
Note how quickly efficiency drops after 60 mph.