I dont know if you can reset that part of the settings mate, but at 19.1 average that sounds about right for a full tank. Its worrying when you do it but when I leave the Nurburgring and head for home with a full tank if I've done 4 laps I usually get about 170 miles showing on the miles to do side of things. This usually increases to around 240-260 before I'm back down there so it will even itself out over a few weeks unless your right foot is perminently nailed to the floor.
Hope it helps mate.
Hope it helps mate.