Why is it that an suv with a 3.5 v6 (montero) gets about the same mileage as a comparible sized suv with a larger - 5.9L V8 (dodge durango).

I have a 98 montero (full size, not a sport) with auto. So far, I've changed to mobile 1 synthetic engine oil, this gave me a consistant 30+ miles per tank improvement. I put mobile 1 synthetic in the front and rear diff, and added a K&N air filter. I see almost no mileage differance with any of this.

So what is the deal with the monty? I remember someone saying something to the effect that the design of it relates to the poor mileage? Any other ideas for mileage boosts? I drive a lot, and with gas pushing $2 gallon, any help will go a long way......