While that might not necessarily be true, speeding is wrong for a number of reasons. It’s against the law, putting you at risk of a fine and points on your licence.
Speed limits are a maximum, not a target. While it might be perfectly safe to drive at 60mph when the weather is clear and the roads are dry, you might need to adjust your speed during harsh weather
By accelerating and decelerating in a smooth and controlled manner, you could expect to save around 20% in fuel.
Focus on the art of progressive braking start slight, increase with pressure and finish light. This allows other motorists to react to your actions and is much safer than constant braking.