Only trails I see requiring 37" or above is rock buggy trails in reality. 40" has to be a buggy trail. To me 37" tires are optimal, but 35s will do about anything you want, just not always as easy. Take the Rubicon, you can do it on 33s with endless stucks and recoveries, with 35s its very doable, with 37s almost too easy for a good driver. Fordyce is the most difficult overlanding trail I can think of that is not just a buggy trail up a dry wash, or even the new slick rock obstacles in California. Those I would definitely want 37s and 40s would rock. But that sure is the minority.