Info not from a Burger but before I bought my 650 I had a Piaggio X9 500.
With throw-over saddle bags I got around 10% less fuel economy than my mate on an identical X9 on a long trip. Without the saddlebags the economy was identical on both bikes.
I have Givi 52 top box and E21 side cases mounted on my 07 650. The bike is as stable with the boxes equally loaded and installed as it is without the boxes on. With boxes loaded and installed I lose some mileage per tank of gas but it still averages 43 mpg that way. Unloaded and without bags I average 50 mpg combined city/highway.
I have a Givi V46 top box and Givi E41 side bags on mine. They seem to have no effect on stability at interstate speeds.
I have done several 500 mile days with all bags and wife at interstate speeds. Traveling at 70 mph, as indicated by my Zumo GPS, (my speedo is about 10% optimistic) I lose close to 20% in fuel economy. It will average 38 mpg at interstate speeds and high 40's otherwise.
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