The two most fuel efficient tall rider bikes are the BMW single and the new Honda NC700X. Either one of those can be hypermiled to a life time of 80 mpgUS. Both are around $7000 in the US. The Ninja650 is one of the most widely available used bikes because so many kids buy them thinking they are cheaper versions of the 600cc supersport bikes and then sell them when they can't keep up with the canyon racers. They are more like a cheaper FJR/ Concours. The torquey engine can be hypermiled to good effect which got me 66 mpgUS. If I were going to ride the bike I would go all the way out as long as would fit on the gearing which would allow me maybe low 70mpg's. Normal, sit up riders, maybe low 60's. The stock bars are really tall. Put a larger windscreen on it and have an upholsterer add 2 inches to get rid of the step in the lowish seat and a Ninja650 would fit 6'6 300 pound guys well with plenty of power to launch them as fast as they want to go.
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