80-100 miles is pretty normal.
It's the sensor, not just the gauge that keeps the top bar on. The actual tank sensor output does not change at all until you've used enough gas to allow it to start falling. It just sits at 0.31 volts output. Then it has to fall enough for the output to cross the threshold that turns the top bar off. I think it takes about a gallon before the sensor output starts to change, then about another gallon before the falling output crosses the 4 bar threshold. That gives you around 100 miles.
This means that whatever clever electronics you added to the sensor, you still wouldn't get any change in signal until you'd burned off at least a gallon of gas. From that point on, the actual sensor output voltage is a pretty linear inverse function of how much gas is left. I actually looked at making an accurate fuel gauge that read out in gallons. It's actually pretty easy to do, but there didn't seem much point to doing it. I ride 200 miles then fill up. Normally I can ride 250 miles and still be safe. If I have to ride more than that, I have around 50 miles left when it starts flashing. That's pretty much all I need to know.
If the top bar never comes on it indicates a faulty sensor. I've read that sometimes they can be "revived" by adding one of the fuel injector cleaners to a tank of gas (e.g. Techron). There's probably some gas deposits gumming up the float mechanism of the sensor.
1990 Kawasaki Ninja 250 (Rebuilt)
2011 Suzuki DL650A
Last edited by BobA; 11-24-2012 at 04:43 PM.