I like this subject...
In my 25,000 km ridden in 2.5 years I've recorded every fill-up and mileage. In my experience, my low fuel warning comes on usually just over 200 miles. I tend to wait for that before refueling.
My 3 best "miles driven" on a tank full is 225. In each instance I needed approx 13.6l to top up...therefore there was 3.4l unspent gas. Ergo, assuming 26km/l or 75+mpg, there was approx another 80kms or 50 miles theoretically achievable.
Those were highway miles - probably 60mph max. I'm running 15T c/s, 46T rear, slightly taller rear tire - all this resulting in approx 10% taller gearing. Also these were summer miles. I was also carrying extra weight - side cases, top case, duffle bag etc.
Living here on Vancouver Island and riding year round - colder temperatures - I notice a big difference in mpgs from summer to winter. Cold heavy air @ sea level, cooler engine operating temperature, winter gas - all hamper mpgs - again, my experience.
Thanks for listening.