I did the tracks usa 121 to 144 extension a couple years ago on a 04 xcsp700. Everybody was saying to gear it down so I did. I'd have to look at my notes to see what gearing I used but I didnt like it. At low speeds it had great power but my top end was slower and fuel mileage suffered. Keep in mind I never did anything with my clutches so maybe that was my problem. Anyway, I ran it that way for most of that first season but then decided to put my original gears back in. After I switched back to originals my great old reliable 700 was back. Great top end, great power. I do quite a bit of off trail and the sled didnt seem to have any issues with belt wear or any other issues. The bottom line in my case was I liked the driveability of the sled much better with original gearing. Modifying sleds (or any machine) is always a trade off. You get gains in one area but then losses in others. Even if you call yourself an off traiil rider you still end up riding a lot of groomed trails to get to your off trail spots so keep that in mind when doing the rebuild. You will be amazed at how your sled will handle the snow after your extension. I put on a 1.5 crossover track and man I can take those huge hills with ease and shoot out of ditches like that sled never did before. The only problem I had was hyfax wear in poor conditions but the ice scratchers took care of that. Good Luck. Think Snow.