Gearing commander can help pick your gearing. Your speedo is probably way off. You need to verifiy it with gps or mileage signs. The tach may be off as well. It is on my Honda. I have an 09 Ninja250 with ZG sport touring screen and stock gearing. I can coax 66 mpg out of it at 65mph with some truck following. I get 87 mpg with my CBR250R on the same commute. Your best economy/ performance trade off should come from running your cruise speed at just below the first torque peak. So maybe 15/ 40 gearing for 6,600 rpm at 65mph. First gear will start to feel pretty long for smooth launches on hills though.
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Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator