I just had a post talking about axles and such, and within that tires were talked a lot about, and most people agreed that to put bigger tires on a 4 banger without regearing, you'll be driving a dog. While some people get away without regearing, they sacrifice that top speed. Plus you are then putting everything else under more stress like your axles and drive terrain. So if you are willing to sacrifice getting there fast, then go ahead and slap on some 31" M/T's, but if you're still trying to get down the highway, then it might not be a good idea without regearing.