Seeing as you live in Sask. your best bet would be 1.25/136. There are no mountains around here and seeing the snow conditions we have had here in the last few years, I haven't seen any snow deep enough that a 1.25 couldn't pull you through. It should also give you great hook up on the hard pack.
I put a 136" x 1 1/2" Camoplast (1999 Polaris RMK 700) track on my ZRT600 to make it a MtnCat. The 136 1.5 is lighter than, and has way more traction than, the factory 121 .875. I intend on putting on a Camoplast 136 x 1 3/16 Challenger for next year.
Screamer, you are thinking "quicker". For "faster" you want less weight, so the smaller/shorter the faster. I agree the deeper lug will generaly be quicker since you hook up harder, but on top end(if all is equal)the shorter track/smaller lug will creep by a deep lug....
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