Yeah, 5% is quite a bit. I remember reading that if you go over or under by more than 3 percent, you have potential for trouble. Not only will your speedo be off by a good deal, but that much more rotational mass will hurt your acceleration, fuel economy, braking performance and possibly other things.
Personally, I don't think I would go with a tire that big, but it does tight as hell! I hope you don't have any issues like rubbing. Please keep us informed.