See my post
on fuel ecconomy about saving fuel.
I get really annoyed with people who drive constantly below the speed limit with no obvious reason to do so. If it is safe to do so one should drive to the speed limit!
Don't forget that most speedos read upto 10% faster than your actually speed, my MGZS180 is no excemption (tested against GPS) so at an indicated 50mph one is actually only doing 45mph, 25% below the speed limit!