I'm getting around 350 (got 370 once by letting it run down to fumes) per tank full with 285/60/18 Terra Grapplers. I've never figured out the percentage that my tires are over the stock tires so I can't tell you how many actual miles per tank that is.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless