I've had fuel gauges on bikes now for 3 years and have found a few things out. I've got a venture that has a gauge and have put 20000 miles on it and never had any problems after learning how it works. Now its a digital readout (bars) but I've found that when it reads close to empty and the fuel light comes on and I park it in the sun at work, when I come out at the end of the day the low fuel light won't be on and the reading is upscale some..... I attribute that to the gas expanding in the sun.... After a few miles, the gauge goes down and the low fuel light comes back on.
Now with the raider IF you look at the gauge really close, what I thought was 1/2 a tank is actually the 3/4 mark. The markings are closer together down near empty. So far when the low fuel light comes on, the tank will take a bit more than 3 gallons to fill which should be spot on with the reserve under the seat. On another note, when I took off the tins to paint, I ran the bike till the low fuel light came on (50 yards from the garage)..... took off the tank by removing the hoses right on the bottom of the tank and not a drop came out.... the tank was empty.
Cliff