I'm going to add my 2cents in here.
First off, if you want your true reading, you should be resetting your MPG settings every tank full, along with the average speed display. Unless you're trying to check what your MPG is for the year or month, you really won't have an accurate sample of what you're doing per tank.
Second, If you've got a v6, your mileage might/could actually be the same or less than what the V8's can achieve. That extra V8 power actually moves the vehicles easier, thus doesn't have to work as hard as the V6. If you put your foot down constantly, you're better off with the V8.
Here at altitude in Colorado, I usually get better in city and highway than my buddies V6.
With that said, my 2005 V8 Sport would pull about 16.5-19 combo, but I actually hit 24MPG one tank full strictly highway. (only once though) That was using miles divided by gallons on paper, not the display, and I was heading east from Colorado to Kansas.
I hope to attain the same or better with my new 07 V8 limited.
On one last note: I have never used Premium fuel in either one of my V8 4 runners. The V8 has never been required to use Premium fuel. This is an advantage over other compitition such as the V8 touareg. Now you want to talk about POOR MPG!
Just my 2cents worth.