if you get the exact same tire and go up to the 75 then you wont notice anything at all... but since you will probably buy a totally different tire, which will have a tottally different weight and load rating, AND go up that tiny amount... you might notice a little change.
I think I lost a little going to the BFG AT KO's in the 265/75... but like Sex-Terra said I would assume it has more to do with rolling resistance/weight than anything. Most definitely worth the sacrifice though!
Everybody's different, but for me I lost almost 1 mpg.
Stock: 265/70 R16 BFG Long Trails #42 (Great road tire, but not tough enough for off road)
Current: 265/75 R16 Yokahama Geolander A/T-S #53 (Best tire ever)
I think the larger diameter and heavier tred are mostly to blame, but rationaly the %mpg decrease is almost identical to the %weight increase of the tire.
Agree with patrick07, saw pretty much the exact same decrease, about 1 mpg.. I also put the exact same tires on, I love those yokos and I got them for significanty less than BFGs. I ran the BFG AT on my pathfinder and they were toast after about 25K