14/59. Stock 125S is 14/64 I think, so that gives, according to my chart, about 8% more speed. My TM stock is 15/61 but I have on there 15/55 which is around 10-11% more. With that it feels too high, very hard to get off the line and sluggish acceleration. Think I'll go 14/55 which should be around 4-5% up on stock. That could be a little short especially with a 5-speed box. Maybe 14/54 would be a better option, but haven't got any other rear sprockets. We'll see how the 14/55 combo works. Mind you, the bigger rear tyre will pull it back a bit too, so the 15 may still work...