In stock condition those sleds can only pull the stock gearing in super optimum conditions (lightly snow covered smooth ice, downhill, dropped out of an airplane, etc.). Thus IMO they need to be geared down even in stock form. For a 136 I would gear down even more especially if you have a 1.5 or taller track. They produce enough power to be fun in the ditches when geared down, but the throttle response is quite slow so they behave more like previous generation sleds. The 2000 PZ500 came with 9 tooth drivers, so switching to 8 tooth drivers gives you an 11% reduction. I went even further on one of them and it is quite snappy given the slow response of that motor. Now if you could get significantly more power out of the motor it would change the whole picture. I think cooling becomes the problem though as you up the HP. They sure are light and fun. If only you could get 120 HP in them without adding any weight.