The CBR125R is probably the best starting point for a fuel efficiency competition vehicle but don't make any mod more trouble than it is worth. Otherwise you will get started and never finish a huge project like that. Find a comfortable head forward riding position by laying on a big, stuffed, tank bag. Once you find a nice, high position, add a taller windshield that comes up just below your eyes. And then make a plank style luggage system or a simple truncated tail trunk which will be more aero and give you large and locking storage. Then you may be able to go up 1 tooth on the front sprocket.
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I can get 90 mpgUS at 50 mph very easily without much tricky riding on my CBR250R. I got 105 mpgUS at the Vetter challenge and 136 mpgUS using EOC at 45 mph at the Green Grand Prix.
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