Just in case this helps anybody. I'm riding a uncorked, stock XR4 at 8000+ feet. Been running a 16/42 and it pulls it pretty good except that on a long hill or against the wind it becomes marginal in 5'th. Went to a 16/39 which allowed me to drop into 4'th for those situations and that works great but 5'th is then too high except in dead calm and/or slightly down hill, but its close to working. I think with a 426 or 440, it would be ok.