I refuse to believe that it can't used down to under 1 gallon in a tank. Sure, the last dregs is one thing, but you're filling up at 8 gallons, that means you're cutting your range to 2/3rds that of the vehicle, you're wasting gas by stopping and then accelerating back up to speed half again as often as necessary, and you're carrying around 4 gallons worth of mass constantly.
It is conceivable that this is a deliberate design, that the tank pump is literally supposed to be constantly bathed in four plus gallons of liquid.
It would, however, be unimaginably STUPID to design such a thing, and therefore if this is the way the thing is designed, and I break it, guess what, GM should pay the price by being forced to replace it under their warranty.
If we baby them by driving around like idiots who strap an extra four gallon weight in our cars for no reason, just to protect their stupendously ill conceived pump design, then they will continue to use that design.
Screw them.
BUT since I've driven cars for 32 years and driven them all down to a gallon or so on 9/10ths my fill-ups, and replaced a grand total of one fuel pump, I really really doubt that this is actually the case.
Surely nobody replaced a functioning system with one that is very noticeably broken, and then just kept at it, with millions of cars on the road all breaking down constantly from this one change. Yeah, I can believe someone did put a pump in one make of car once upon a time that was stupid, car companies somehow manage to do this sort of crap all the time. But market forces would rapidly have corrected that problem, a better pump design would go in, or else they would revert to the older pumps that didn't have the problem.
Therefore, cool rational logic and outraged sensibilities both suggest I should drive my car like every other car; fill up when I get really low, OR whenever I happen to already be at a convenience store and am low enough that I figure "might as well, would save me a stop later"
IF the pump needs to be bathed in gallons of liquid at all times, then the simple trick would be to mount it in a separate sealed tank full of liquid, not in the gas tank. Took me roughly 1.2 seconds to think of that solution, automotive engineers have all their working lives to think of something better. If they haven't, it's the fault of people like you who let them get away with this sort of half-assed design work.
(except that, again, I don't even believe it is anything but a combination of memories of an early bad design coupled with the persistence of outdated good advice that no longer has a place)
None of which has anything to do with the OP's question, which is a simple mechanical one; at what point does the low fuel light come on in a Sonic, is his broken... answer is somewhere during two bar, if it isn't on by one bar it's being funny, and his pic of zero bar and Still no light is definitely abnormal behavior.
