I've run 31's with stock gearing, you will just not be using 5th gear so much since it will be to high geared for that. Other than that it will have no effect since Samurai's are slugs to start with. The one thing to consider for an easy fix is to drop a 1600cc engine in. That will help a lot, you can also drop a Sidekick/Swift engine but that takes more changes than the 1600.