Honda has improved the PCX scooter this year. They added displacement to 153cc. This along with other minor engine tweaks has raised the top speed to 70 mph and makes the bike legal for California super highway use. I believe that streamlining and a slight, although difficult change to the final gearing such as fitting a 15 inch rear wheel or maybe even just finding a slightly larger tire would allow this machine to hit 80 mph. The engine would certainly be the most fuel efficient turn key powerplant to choose from. The only question being the efficiency of the CVT trans.
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2013 PCX150 Overview - Honda Powersports
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