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Actually to change the torque or rpms for a given mph up or down it’s better to change the rear sprocket since going to a smaller front sprocket requires the chain to turn at a smaller radius causing more friction and wear. It’s always a good idea to change the chain if it's not new when changing sprockets and visa versa since these surfaces are made to wear together with respect to the chain roller.
The 09 comes with a 13-38 and we noticed a dip in torque at low third. A 5-6% change down would be good so,
38/13= 2.92 stock sprocket ratio
40/13 = 3.07 new sprocket ratio
(new ratio / old Ratio) - 1) * 100 = change as a percentage.
(3.07/2.92) - 1) x 100 = (+ 1.0513 -1) x 100 = 5.13 % change from stock down.
Positive % trade top for bottom, negative bottom for top.
I could figure this out better if I knew what rpm I had at low third but I can try this and experiment without a dyno. 5-6% changes are good to try at a time, any less won’t be felt, any more might be too much.
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