Heartless speed cameras aside, excess speed can cost a serious amount of money. The amount of fuel required to maintain a speed increases dramatically with each increase in speed.
To achieve meaningful savings, try not to think in terms of vehicle speed though. Match a speed to an engine speed (keep note of the gear!). If driving at 70MPH requires an engine speed of 3000 rpm, set 3000 rpm as your target. The aim is to keep your engine speed consistent, and too many people focus on the vehicles speed leading to excess acceleration/deceleration.