you're not helping me here

At this rate I'll have a 5.0 on order.
My wife had a 3.0 E53 X5, then a 335xi, and now a 4.8 E70 X5. I had a 740i, then an E60 550i. We've never had economical cars when it comes to gas mileage. But the current X5 is different. The gas gauge moves at the drop of a hat. Go easy on the throttle in D, no revving and you're still not going to get 60 miles out of the first 1/4 tank. The gauge actually moves off full by the time I get home from the station a few miles away - seriously! Now if this was an M capable of belching out massive low end torque then it makes sense, but the 4.8 isn't even that fast (especially since I had the same engine in my 550 which was a rocket). In fact the 35i is just as quick. So the poor mileage of the 4.8 just isn't justified in my opinion. Now the X6 5.0 is a much faster beast. A genuinely fast car. But I just don't know if its prudent to get one since our current mileage with the 4.8 annoys me?
Last thing complicating this decision is that the leasing is better on the X6 V8 than the 6, so you get more car for the buck....?
DRP
Thanks for the continued feedback