My last tank yielded 20.3 MPG with about 75% highway. Half of that was at 80 mph and the rest war around 60 mph with some mountain driving.
Overall I've been getting between 18.5 and 22.5 depending on how much of the tank was used on the highway versus in town. I've noticed that it does much better with moderate highway speeds.
For those of you running the 265/70s, your tires are about 3.5% bigger than the stock 265/65s. This means that you need to adjust your calculated mileage by 3.5%. (Alternately you could adjust the miles you use in the calculation by 3.5% -- the result is the same.) So, if you calculated 20mpg, you need to multiply that number by 1.035. 20 * 1.035 = 20.7mpg. This should make you feel a little better.
I would expect, though, that the larger tires are getting worse gas mileage due to extra rolling resistance. On my previous SUV, I lost 1mpg when I went to a larger and more aggressive tire. I'm sure that both aspects contributed to the mileage.