I lost 2MPG when I went from 33's to 35's so you should see a gain there. Go 285 as there are many more tire choices and if you have an issue on the road you have a better chance of finding a replacement on the shelf.
Keep in mind your odometer is not going to be right with any tire size change. With 35's you would actually be going farther than the odometer says. That being said IMHO I would not put anything smaller on this big of a vehicle. But again it all depends on what type of driving you do most. One other thing to keep on mind, is you do get a better ride off road with the 35's aired down.
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