Upvote:12
Fuel gauges are electromechanical and prone to friction (though they don't normally stick as badly as I've had in my old van). They're certainly not precision instruments. The low fuel light seems to latch on until you've put more in, at least in some cars I've owned, probably to avoid it going on and off as the slope of the road changes.
But you'll get worse than the 30mpg you quote for two main reasons:
Fuel is far cheaper than the likely costs of running out - you're likely to be charged for the car having to be dealt with, let alone if you miss a flight (a common reason to be hiring a car). I wouldn't chance it, and fuel here costs about twice as much as in the US.