I had the stock tires on my gsr and switched to some 205 50's. Would that effect my speedo at all? Or not enough to notice?

 

it wouldn't really affect your speedo, you would probably hardly even notice the difference.

 

Since the 205/50/15" tires are a touch smaller than the 195/55/15's that come stock your speedo will read a speed that is faster than you are really going. The difference will be very slight but the faster you go the further is will be off.