http://www.fueleconomy.gov/feg/driveHabits.shtml
While each vehicle reaches its optimal fuel economy at a different speed (or range of speeds), gas mileage usually decreases rapidly at speeds above 60 mph.

You can assume that each 5 mph you drive over 60 mph is like paying an additional $0.30 per gallon for gas.

Observing the speed limit is also safer.

60 MPH -> 96.56 Km/h
30 MPG -> 7.84 l/100k


http://auto.howstuffworks.com/framed.htm?parent=question477.htm&url=http://tqjunior.thinkquest.org/4116/Trip_Planning/speed.htm



http://www.greencarcongress.com/2006/05/fuel_consumptio.html