| Hancym, changing from a 38 to a 39-tooth rear sprocket will raise your engine speed at 120 kilometers/hour by 140 RPM. Not worth the effort or expense in my opinion. But, you can probably get away with using the same chain if you also raise the front end to match the ecpected increase in rear ride height. However, you'll also get a shorter wheelbase however that tends toward decreasing high speed stability. | 
Shazaam!
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