In theory (which is all we can hang our hats on with H-D, since they aren't terribly consistent), the fuel tank is rated to hold 6 gallons when filled to either one or two inches
below the bottom of the filler pipe, I forget which. So if you fill like most of us, up to the bottom of the filler pipe, you would expect to get more than 6.0 gallons into the tank. Other's have posted in the past that they could get more than 6.0 gallons in their tanks, so I'm surprised that you could only manage 5.76 gallons.

Was the tank actually dry, or perhaps was there fuel remaining that the pickup didn't manage to suck up? What I'm trying to say is that you might want to verify the actual capacity of that tank and make sure there isn't some other reason the engine ran out of fuel when it did. Drain it
completely, then fill to the bottom of the filler pipe and see what you get. If it still comes up less than 6 gallons, something isn't right and you might want to discuss it with H-D. As for the accuracy of the gauge, that's another of the many things Harley passes off and expects customers to live with. If they won't fix it, you or someone you hire can at least make it read more accurately in the all important bottom half of the range by adjusting the float rod or making some electrical tweaks.
Jerry