I think you have your #'s backwards. The 13/40 should be your internal final drive gearing which comes to a 3:08 ratio. The 31/16 should be your sprockets. Both are the best you will do. Going with a smaller ratio than 3:08 will help top end just like a hi way gear in your car but bottom end will suck. Increasing your axle sprocket will give you better bottom end but you will loose top end. The motor needs to produce more power. For more power you need a good quality performance head that has the large runners and larger valves and a good performance cam. For more power a proper port and polish on the new head. A big bore kit will than enhance the power output.
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