Ive just skimmed this topic so pardon me if this has already been mentioned..
When you step up to a bigger tire, from a 245/70/16 to a 285/75/16, of course you loose mpg, your odometer and spedometer are off, you loose power high and low-end, but most importantly, your severely taxing the longevity of your drivetrain, namely your torque converter. Without regearing, your torque converter must work even harder to turn the tires that are larger and require more momentum to change their velocity (from stop or from one speed to another, hence "velocity" means direction and/or speed).
My only recommendation is, if your going to do it, do it right - really try to get it re-geared, you will be happy with the outcome. tons of guys lift their big full sized vehicles 6-8" up and throw 35s and don't spend 2 grand to gear em, in a year from now, they all spend 2 grand to re build their tranny, then 2 grand to re-gear, when they could have saved themselves from an early tranny rebuild.
I've got a 97 4runner with 245/70/16 Duelers. I just bought the vehicle a week ago and noticed it had duelers on it like yesterday lol. They perform good on road, can't say much off-road except I had to throw it in 4 to get up a climb that I did in 2wheel with the BFG ATs. I don't really like the look of the duelers, the distinctive sidewalls of the BFG AT look really bitchin tho IMO =)
if you want some, ive got 4 Michellin LTX A/T tires, they're kind of old, but have about half tread.. ill sell em to you for 75 bucks lol, all 4 of em.The date on them is 128, 12th week of 1998.