Interesting post, as it happened to me yesterday; I left the car running while filling, and afterward wondered if the vapor recovery system was unhappy with me. I'd thought it was full, but the gauge never went past 3 1/2 bars, so when I got home, I looked at the receipt, and I'd only put in 4 gal. Must be a common problem, eh?
Ive noticed that when i start the car on 1-1.5 bars the light comes on but after a bit of driving the light goes away. The gas light is to warn you that you are on your reserve fuel and when the gauge hits 0 it means you will be empty.
I dont really pay attention to the light except to remind me that i will need to fill up soon. But i have gone at least 50 miles with the light on.
A lot of the gas pumps are too sensitive and cut off the filling too early. Many in my area won't even let me pump 1 gallon of gas on an empty tank. I just estimate how much I think the tank needs, and keep regripping the pump handle after it shuts off until I get what I think I need. I have always been able to get all bars on the fuel gauge this way.
I read an article last year that stated the reason many gas stations have pumps that cut off too early is because they don't properly maintain their vapor recover systems.
I have about 1 bar and can put in about 8 gal when the light comes on. If the car is tipped to the passenger's side I believe it will read high, and with a low tank you are in danger of running out of gas, as the pick-up is on the drivers side..... for a Fed type 111.