First, I am 100% a nub. So, take this with at least a few grains of salt.
I just put on 265s. No spacers and some almost free crap wheels.
But I do have Radflo (set to about 2 1/16th up front) and MarkMC on back. Without that lift, even with removing the plastic lining, you may hit the wells on turns. For example, with some black steelies and wheel spacers, I would have needed a body lift (or some aggressive trimmming) - at least when I tried it yesterday.
I will be pulling out the plastic wheel lining tomorrow because the bolt to the rear of the front wheel well will hit the tires when compressing at speed when turning hard. With spacers, you may have even more trouble as the further you push your wheels out from the pivot, the more they will hit your wells, particularly the back of the front one (at least on my 2005 XLT)
For speed difference in sizes:
http://www.offroadtb.com/articles/tech- ... alculator/For mileage, I lost a bit.