The longest tankful recorded here for my 1995 R1100RT is 424 km (264 miles) from 25.2 litres, with a couple of others over 400 km.
R1100RT (BMW R1100RT) | Fuelly
Sadly I think everything recorded there is pretty much short distance city work. Back before I started using fuelly I often went from Wellington 880 km to just north of Whangarei with a single fuel stop at Tokoroa, basically exactly 440 km (273 miles) for both legs. And never going slowly :-)
I once toured 2-up with another couple on an F650. While following them around slowly (100 km/h) I got 535 km (332 miles) from one tank. It's extremely rare for me to ride so slowly on the open road, but that's what the bike will do.
I'm sure the newer ones (1150, 1200, 1250) are more efficient.
More recently I've just bought a CRF250R Rally and my sister an NC750x. Will be interesting to try riding together sometime. My first tank I used 3.5 l/100km but that's new tight engine and some increasingly thrashy riding to break it in. The 750 can supposedly get that kind of economy too, but has a 14l tank vs my 10l. Fuelly shows most of them averaging 3.8 or 3.9 l/100km real world, which probably translates to 450 km (280 miles) maximum practical.
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