For those of us that take long trips routinely, or have long commutes and don't want to stop for gas every day, I think the miles per tank is a more relevant figure that mpg... but its just math so no big deal. The OP intends to drive the V8 the same way he drove the V6 and simply wanted an idea of how far he could drive per tank. So the relative miles per tank is equivalent to the relative miles per gallon, given that the tanks are the same size.
Once your add in other variables, like driver, ride type.. etc the comparison may become irrelevant.
Good thing I got that lifetime warranty, the way I abuse that fuel pump
If he never lets it get below say 1/4 tank before filling, and you let it run dry, how is that an equal comparison? You may say "I get 500 miles per tank" because you run it dry. If he in turn tries to do the same thing, and only gets 425 because he fills up at 1/4, he's not going to be happy, and will then complain that his Jeep gets horrible mileage, when in fact he has no idea what mpg he's gotten.
Or one guy might get 300mpg per tank becasue he does all stop and go city, while the next guy gets 500 per tank becasue he does almost all highway.
So that really tells you nothing, only that you get better mileage on the highway.
Only good comparison is MPG. But yes, you are right, driving style, type of commute, road conditions, weather conditions, all make a difference in mpg. But it's still a better figure to judge by.
If you get 500 miles per tank (based on a 25 gallon tank), but have to fill it with 23 gallons, you've gotten 21.7mpg. If you've gotten 430 miles per tank, but have filled it with 18 gallons (at aprox 1/4 tank mark), you've gotten 23.8 mpg. Sooo, the less miles per tank driver actually got better mpg. See my point?
And if you take frequent long trips or whatever, what's the difference? Yeah, I have a long commute and don't like stopping for gas everyday either. SO, when I need gas, I FILL IT UP. If you're one of those people that only puts in 10 or 20 bucks at a time, yeah, you'll never be able to really figure out mpg.