I strongly reccomend visiting the jt sprockets site catalog (to identify part #s for your bike) and buyers guide:
JT Sprockets: Catalogue
FYI, I went from a 47 to a 37 rear on my little gs without any problems. Off the line is slower but hiway rpms are way down as expected. I've had 100+mpg tanks, but I don't need to go 75mph either.