Saving fuel by uncoupled?

Nen buddy of mine arrived today and said, If you let the car roll out you save sprit. So it’s logical!! But now he wanted to know what would save you more sprit? So I mean jetz, whether uncoupled or left in gear. So, my thesis: of course with uncoupled gears. But now someone else meant that the current consumption of his car in the disconnected gears is higher than with coupled gears. (That’s what his board computer told him)