Coasting along

There's a hill that I drive down daily to get to work, between the length of the hill and the speed picked up coasting down it, there's no need to touch the gas pedal until you've driven at least a mile, when you speed finally drops below 40, the ususal spead of traffic on this road. You can go even farther if no one's behind you, dropping to 30-35 before you have to stop for a traffic light.

My car gets about 30 mpg.

what % of gas is used by a car when coasting, as opposed to driving on level ground?