That would IMO be huge improvement. My first concern is front fitment, No sure how it could be done in front without a rather unfavorable offset and/or restricted turning arc, but if possible I would do it in a heartbeat. Second concern, it seems to me tire availability is every year becoming more narrow, meaning what you can buy this year maybe no longer be available in the next year or two, and lastly, is 11"W may not be the most favorable width for 315 or at least right on the edge, not sure. So is there already a large selection of suitable's 315-30x18 currently available to chose from?

Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.