I'm running a 99 thundercat 660FT and would like to know the best gearing with good traction. I've got some aftermarket parts installed and I running about 190hp at the crank and I way 250lb's dressed. I'm thinking 21-39's or 23-40's. Any help would be great.

I've always ran 20-39 gears. I like the 20-39's because you have a good mix of stump pulling power and top end. What pitch chain are you running?http://www.snowmobileworld-s2.com/forums/iB_html/non-cgi/emoticons/confused.gif
If you're drag racing though I'm not sure what you would want.....

I am running 24-39 in mine right now, going to try a 23-39 tomarrow(running 660&#39http://www.snowmobileworld-s2.com/forums/iB_html/non-cgi/emoticons/wink.gif * *I used to run about the same amount of HP(192HP). If memory serves my correct i got the best results from the 24-39's. * What helix, springs, and weights are you running? * Next time you are out doing a run, take a quick look at your speedo to see where its at(not very accurate, get a hold of a radar gun as it will be much better) and then just figure out the gearing from there(i usually run 10-15MPH more than i will reach on mine, can get away with more on 600's and stuff, but seeing as how you owned your sled for a little while you prolly know it weighs way more than a 600 :-) One other thing to try is to make a line on your prim. with a marker and see how far your belt it going up the sheeve, a little over 3/4 way is preferable(full shift isn't very efficeint). * Anyway, if you want more info just ask. Ill take a look to see what I had in there last year.

Yep. I have had good luck setting all sleds the same way 1000-IMP posted. I shoot for 10 mph over what I can hit & check the shift on the clutch face. 23/40

22/41 is the way to go. Better longevity for the chain.

If you want a fast setup for 660 drags:

Primary: Polaris 10M62 weights (slightly file bushings is needed)
Cutler Purple/white Spring

Secondary: HPE Green High Tension Spring
62/56 Helix

Run 22/41 Gears. GET RID OF AIRBOX!!!

That is what makes it run. Period!

tried the 23/39 today, stick with the 24/39 * i found it to be better. I am however a hundred pounds lighter than you stoneythunder, so that might make a difference. * Did you try the mark on the clutch thing. Id would be nice to know how far the clutchs are shifting.