The tank is odd shaped so you probably were not really seeing the bottom of the tank. Maybe the bottom of the top part of the tank?
Unless it's been adjusted by an owner before, as Satan said about the top 4gal reads from E-F normally.
The gauge should go a pretty long way under E also, so it's like reading 5/4's of a tank sort of. 5th being 1/4 UNDER empty. My 88 did that so I read the Under empty as part of the gauge. Not sure about my 95 as I seldom let it get that low. You'd just have to check if yours does that.
Need to learn to read your gauge or adjust it, or both. Sometimes the MPG can vary drastically with riding style, winds, bad gas, etc... so relying on mileage rode is not always good either.
My 88 would do about 230 miles to a tank (37.8mpg 6.3gal tank) if I pushed it to empty which I never did, but once I ran out at 180 miles and was lucky I was turning into a station right then and coasted downhill to the pumps.
Same with my 95, almost got stranded in desert when MPG dropped from normal 40-42mpg to about 30mpg! Just that one tank of bad gas then went back to normal MPG again.
Fighting really hard head winds at high speed can also drop MPG ALLOT.
If the low fuel light comes on (sometimes they don't work) your supposed to have about 1gal gas left I think. So how far you can ride depends what your MPG is right then!
Though I could push it longer I normally try to fill up around 180miles, or sooner if want to stop for anything else about then. Both times I almost got into problems was right at the 180 mile point also! Strange because there is one trip I make that I often ride 205 miles on one tank because the gas is more expensive there and I avoid it. So I fill at home, ride 205-210 miles and fill at home again.
The electric fuel pump is in the tank and the gas helps cool it so not running to low much helps the pump live longer they say. That makes sense in theory though I've not pushed it to the test LOL