Although not recommended, I deliberately ran the tank empty exactly one time, while carrying a gallon of gas on board, to see how much usable gas I really had. The tank does, indeed, take 3.6 gallons. I almost always have a GPS on board, and reset one tripometer with every fill-up. Multiple measurements show that the odometer is about 4% high, although the speedometer is about 10% high.
By strange coincidence, the MPG reading is about 4-5% LOW compared to usual actual gasoline used for INDICATED mileage. However, if I adjust the INDICATED mileage to what my GPS says is the ACTUAL mileage, then the MPG readout is about right! I think it's all entirely coincidental (although I cannot understand why the MPG reading does not sync to indicated mileage), but the bottom line is that MPG reading turns out to be about right if you're using GPS mileage. Strange, indeed.
I've been measuring my MPG cumulatively since the beginning of 2016 (about 7,500 miles ago) and am averaging about 62 MPG using actual fuel and indicated mileage. Adjusting the mileage reading for the 4% error, I've been getting about 59.5 MPG ACTUAL.
So what does this all mean for range? If I'm doing my average riding (which excludes extended expressway speeds), then I can expect about 225 INDICATED miles of maximum range. I make sure to buy gas no later than 200 after I've filled last time because I don't want to run the tank dry again.
For reference, I'm 165 lbs. and keep the rear tire at 30-32 psi most of the time.
And, yes, I'm retired. Who else would have the time or inclination to do all this obsessive measuring?