I just finished changing the sprockets on my Z50R with the 125cc engine. and plus 8in extended swingarm. There are some very steep hills on the technical trails that I ride and I needed the max extended swingarm I could find in order to avoid wheelies under any circumstance. In addition I swapped front sprocket from 14 to 12 and the rear sprocket from 37 to 40. NOW max redline in 1st gear with 125cc motor is 7 mph. Max redline speed ( without a tachometer ) seems to be about 30mph in the fourth gear. When riding on the road or level surface it is most convenient to start off from dead stop in 3rd gear. But boy will it chew up a hill in 1st gear. AND if starting in first gear from dead stop, just leave the throttle at idle and the engine will not die when you let the clutch out. So now it will be very hard to stall it out on very technical slow trails. .