I have 16/45 on my FS650.
It's good in that it doesent leave any gaps in the lower gears. The 5th/6th gap is not large enough to matter much.
Gap = a sustained speed at which you can't really ride it without either revving it in one gear, or lugging it in the next.
It does 70-75 mph just cruising along without revving too much. At 65-70, I can shift down a gear to overtake a car, and might hold 5th untill 75..
I swap the front to a 15 tooth for dirt riding (trails). It works well and can climb ugly trails well. It really gives you two gear choices for most conditions.
It (15/45) would be decent kart track gearing if the back straight is a short one (<80mph)
For a road course circuit, you would probably want something like 16/42 unless the straights are a half mile or longer, then the factory gearing of 16/40 would be probably be best.
