Originally Posted by BuckMoto
If you are thinking of jumping 3 teeth in the rear from a stock 42 tooth to a 45, you might consider simply dropping a tooth in front instead. The gearing would end up nearly identical plus you can alternate between 1-tooth different countershaft sprockets to suit your ride and only need a chain adjustment. Adding 3 teeth on the rear would probably require more chain links.
17/45 = 2.647:1
16/42 = 2.625:1
16/45 = 2.8125:1
15/42 = 2.8:1 (my current gearing)
I have had no issues with the 15 tooth countershaft (especially now that I have a straight swingarm!).
That 16/42 and 17/45 are super close on the ratios. I can tell as much difference between a new tire and a worn tire in RPM/MPH/traction, as 16/42 vs 17/45. 17/45 had a tiny bit more grunt/bite than the 16/42 does. I am presently 16/42 with a 17 on the shelf for slab trips, after running the 17/45 for the last set of gears.