If I travel above or the same as 110 speed limit, do it have any improvement or do I gain?

Yes, especially on freeway 110 and above will definitely be wonderful of course light on the pedal. If you push the pedal all the way you are definitely wasting more fuel that is one of the rules of saving fuels even without the unit. It should be 30% and above of course depends of your car model, and your car engine condition.