I ran my 2015 Roadster out of fuel once, just to check usable tank capacity. I had a spare quart of gas with me, as I usually do, so was able to get a fairly accurate reading when I filled up. The tank holds 6.3 usable gallons, to the bottom of the tank's "neck basket", when parked on the side stand.
The bike upright, usable capacity is 6.45 gallons, filled to the brim.
When the yellow "low fuel" light comes on, I have about 2 gallons left. My "miles to empty" reading jumps from 14 or 15 to "0". When that happens, I have about 1 gallon left.
I typically get 36mpg (according to the odometer) on my daily commute. 36mpg x 6.3 = 227 miles of range.
I just fitted a Bridgestone Exedra Max 240/55 rear tire, which has a 3-4% larger circumference, so I am expecting to get 34.5 - 35 mpg according to the odo, which counts revolutions of the prop shaft. In reality there will be no change.