If you are in between a rock 18t and a hard place 17t you can always fine tune your final ratio by replacing the rear sprocket.
The stock rear sprocket is 42 tooth. With a 17t front your final ratio is 2.47 If you jump to an 18t you final ration is 2.33. If you want a little closer to the 18t move up to a 43 tooth rear. This will give you a final ratio of 2.39 If you want to stay closer to 17t ratio. Keep the 18t front but put a 44 tooth in the rear. This will give you a final ratio of 2.44
I also wonder whenever sprocket sizes come up nobody ever talks about changing the rear sprocket. Every bike I have I usually only change the front sprocket once and fine tune with the rear sprocket.
Fryguy
