Ran my 06 with a 5 gallon tank till the light was on for about 14 miles, fueled up with 5.2 gallons. either the pump was not calibrated or reserve and pump both wrong.
Ran my '05 with the 5 gallon tank and the original tank of gas down to just slightly under 1/4 on the gauge. When it began missing as I sat at a traffic light, I pulled into a conveniently located gas station and filled up with 5.4 gallons of gas, with the bike on the jiffy stand and fuel just up to the three little holes in the filler neck. After the incompetent dealership couldn't fix the gauge, I fixed it myself. Subsequent tests confirmed that the capacity of the tank was in fact 5.4 gallons when filled as I described, btw. Harley measures capacity at a certain distance below the filler tube, so if you fill it all the way to the bottom of the tube you will get more than the stated capacity.
When the gauge is adjusted properly, the low fuel light should come on at approximately 3/16 (1 hash mark below 1/4) and you should have approximately 1 gallon of fuel remaining. After adjusting mine to that standard several years ago, it has remained consistent at the low end of the scale and I have no qualms about relying on it. If I fill up right after the gauge hits 1/4, which is my normal practice, it takes 4.0 - 4.2 gallons to fill it. Meaning there were 1.2 - 1.4 gallons remaining, exactly what I would predict based on the gauge reading. In case anyone cares, when I originally adjusted the gauge I also checked it the other way, by adding 1.0 gallons to an empty (drained completely) tank and checking the gauge reading. That's the best way to do it, especially if you have one of the current models that is hard to get full.
It's highly unfortunate that H-D doesn't think it's important enough to spend another buck or so to give us a good fuel gauge system that everyone can rely on right out of the crate. I don't know about the rest of you, but I stopped using the tripmeter in my cars to decide when to fill up about 35 years ago, and I haven't come close to running out of gas in any of those vehicles. Didn't have any such problem with my Yamaha's or Honda's either.
Jerry