I'm not sure how many gallons I'm putting in. But when I fill up at 100 miles on the trip meter.................. I'm on empty. and yes weight is weight. I found a set of the aluminum drums on a race car at a local race track I tech at a few years back.
I realize this quoted post is old, but the fuel gauge on the 1500s is badly off in calibration.
I fixed both of my 1500s by removing the fuel pump, siphon all gas out, pour in exactly 3.0 gallons of gas.
Now, handlebar kill switch off, turn on the Key-switch and watch the fuel gauge.
reach inside the tank to the left front corner and feel the float arm. move it up or down and notice how the gauge reacts. you want to bend the arm so that the gauge will read exactly 1/2 tank.
The effect is that the gauge will now be very accurate all the way to the empty mark. when it reaches the bottom line, the low fuel light should be starting to burn.
The top half of the gauge will be slow to come down off the top, but who cares? what you really need to know is how much is left in the tank at the bottom end of the gauge.
In your situation, I am willing to suggest that at "empty" on that gauge, you never, ever pumped in more than 4.5 gallons of gas.