It hampers overall performance. Think of trying to climb a double ledge. Front tires are just over the edge of the upper ledge and the backs are up against a 12" tall ledge. If you increase tire size without increasing wheelbase, you are effectively shortening the wheelbase because the distance between the front and rear tires is shorter. That makes it harder to climb stuff that you would normally walk right up since the rear tires won't let the fronts get up on top.The part I don't understand is why there is relation between tire diameter and wheelbase. More than one experienced builder has given basically the same info so I don't doubt the guidelines. Any thoughts on how tire size and wheelbase work together?
I also have the advantage of being around a lot of rigs that were built for 37's and then the owners figured out they could toss on 40's with a small bit of trimming. Across the board with one exception, they all sold the rigs shortly thereafter due to how crappy they worked.