According to the manual, page 3 - 12, "Fuel reserve amount (when the fuel level warning light comes on): 6.7 litres, 1.47 imp gallon", so there's nothing wrong with your fuel gauge. It just seems like Yamaha being a bit conservative. Personally, I'd like to re-calibrate it so the last bar flashes with 2 or 3 litres left, I think that would be more realistic. As it is, you could probably do about 70 miles on reserve, with an annoying light flashing at you!

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-- Triumph Explorer 1200 --