Hey tyv12 maybe you are better at searching the I, but I have looked at a lot of threads and found a lot of talk about fit and lift but not much specifically about mileage and power while towing with an Auto. If you don't have anything to contribute it's ok to just go on to the next thread...
No offense to anyone involved at the moment, but don't get too put off by the search nazis. They exist on every forum on every web site. The problem with the search function in general, no matter where you try to use it, is that most of the time it's dang near useless. And that's because most threads, just like this one, stray from the topic at hand on average after about the 6-7th response. This one started to stray ironically enough, with the "use the search" comment. Threads are about 15% info and 85% a bunch of non-related crap (just like this post) when it comes to the topic the OP is looking to research.
The best use of your time, by FAR and much to the annoyance of those in love with the search function, is to do just what you've done and that's ask. Either someone will step up with the info, or they will have a link directly to the thread the search nazi thinks you will find on your own, before going slightly bonkers from sifting through all the chaff. Some folks get down right agitated when they think a question has been "asked to death", yet can't help themselves from jumping in, reading the question AGAIN and chastising another poster AGAIN, instead of just scrolling right on by...
You'll be going up about 2.5" in overall diameter. Clearance is liable to be an issue unlifted, especially off-road, but you might get away with it on the street. I'm running 265/75R16s at the moment on a 2010 DCSB TRD Offroad and dont have any fender clearance issues, but I know if I bump up to the 255/85R16s I have in my garage it is going to be very very close at full lock, even on the street.
Mileage and power are only a couple of things you'll need to be concerned about, especially since it sounds like you'll be off road. Yes, they will suffer. How much depends on what type of driving you are doing and how much you are pulling. I'm going to mount the ones I have and see how it goes in the power/mileage dept, but I have no illusions about being able to do it without a lift. I expect to have to do it, period. Power I am less concerned about since at the moment it isn't expected to do any towing, but if/when I get to that point, I expect some new gears to be part of the picture.