I used a Suzuki OEM 16-tooth sprocket from mid-80's dirt bike as per the advice below: "How can I reduce vibration at highway speeds? Exchange the factory 15 tooth countershaft sprocket with a 16 tooth (Suzuki part number 27511-37200). The last time I purchased one it cost about $16. Note that this sprocket was originally used on a mid-eighties Suzuki motocross model and is designed for #520 chain, rather than the GR650's standard #530 chain. That means it is slightly thinner than the original sprocket. My experience has shown no problems with excessive wear when used with the #530 chain. Since the sprocket is thinner than standard, you will need to add a washer between the sprocket and lock washer when installing. If you don't, the sprocket will remain loose on the countershaft splines even after you torque the sprocket nut. With the new 16/38 gearing, expect 4700 RPM at 65 MPH verses slightly over 5000 with the stock 15/38 sprockets - with no noticeable loss in performance or problems clutching off from a stop light. At 65-70 MPH, you will be below the point were engine vibration and valve train noise becomes an issue." http://tcmcstuff.tripod.com/GR650FAQ.htm Worked on my bike, but I think I paid about $6.00 more.