Are you talking about empty when the last bar on the fuel gauge is still solid?
I find I can really still go quite far before the bar starts flashing and the trip odo switches over to zero to show the kilo's traveled on reserve. Then I can still do about 100 km on the reserve which is about 6.7 liters.

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2014 XT660Z - Barrett pipe, DNA stage1.5, Barkbuster hand guards and handlebars, Pivot pegs, Teknik suspension, Guard-it engine protector combo, B&B rear rack, Trax topbox.