I was running 35" on my SMB. I actually went down in size to 34". The most noticeable change is the wander it has now. It likes to wander at highway speeds much more due to the almost 2" difference in width of each tire. I had no issues with rubbing. I also have the SMB 4x4 system. Mileage seems to have dropped slightly maybe because I went from a AT tire to a MT tire with more aggressive tread patterns? Too hard to tell if mileage is really affected. Best thing to do is post this at the SMB forum.
If you have wander in highway NOW with MT profile and before it ran straight with ATs mounted: this is for sure due to the profile/softness and not the diameter!
A diameter change in that low range doesn't change a lot, if suspension/set up is technically OK.