[JQmile]

If I'm thinking right, think of your engine as a giant air pump, because without fuel, that's what it is. When you're idling, your throttle valve is closed, and there is a restriction. When you are revved up, your throttle valve may be open slightly more to keep a steady 3000rpm, but your engine will be revving higher and creating more of a vacuum, hence your readings will be similar. In theory the higher revving engine should use more fuel, but probably not that much more. Remember that engines run based on air:fuel ratios, and with the throttle mostly closed the engine won't suck in that much air, so its fuel economy won't suffer. But it probably won't be any better either.