150 w/12's

go grab a couple jt front sprockets the next two sizes bigger that will get ya in the ballpark cheap.

If you weigh under 200 lbs 45 tooth. Over, 47 tooth. That's with stock 15 front counter sprocket.

I run 15 40 w/12`s

Wow. I've heard of little guys running 43 rear but 40? How much do you weigh? Are you on a 150R?

Underground is fat and slow, don't listen to him.......... I run a 16-43 on a CR85........ Same as a 15-40.

Yea, but that's with a Two smoke. I think you'd suffer getting out of the corners hard with CRF150R with a 40 on the our tight go cart track.

If you ride short tight tracks you are probably better with the shorter gear ratios. However, 4 strokes allow you to be in the neighborhood of correct without punishing you like 2 strokes do. It will also depend on how fast you are not to mention, how early and how quickly you pick up the throttle coming out of turns.

Longer gearing will sometimes force you to become faster through turns so you don't lose momentum and drive. Steep learning curve though. lol
These guy probably have you pretty close. Play around and see what you get.
Good Luck!!

i have made the switch to 12's recently on my 150R. i was initially running a 15-47 and it was ok at tight kart tracks but not enough gear for Road America's supermoto track. it did work at Shawano but just. i was just touching the revlimiter in 5th on the back straight.
i got a 16t front and it is perfect at Shawano now. i have more flexibility in what gear to be in and i'm not on the revlimiter at all. i have not been to RA since but will in the next week or so.
the 16 is sold by sprocket specialists (they had 3 in stock when i got mine) with a disclaimer that u have to mod the case to make it work. not so. u do have to remove the cover and the case saver chain guad and some of the swingarm buffer but it does not contact the case.

150r

:thumbup: thanks for all the in put