I've copied and posted my own answer from a thread which appeared on AB only last month:
"When I started 'trade plating' (delivering vehicles around the country), I didn't initially have a fuel card, so I had to put fuel into vehicles at my own expense and then claim it back. As I'd been unemployed prior to that, I had barely enough money to buy food so (obviously) I wasn't going to put fuel into a vehicle's tank unless I really, really had to!
I soon learned that there was no problem driving a vehicle for at least 30 miles after the fuel gauge showed 'Empty' and the warning light had come on. Other drivers told me that they'd often got away with driving more than 50 miles with the warning light on.
Manufacturers know that their vehicles will be driven in, say, the highlands of Scotland (where it can be dozens of miles to the nearest filling station) or in far remoter areas, so they have to ensure that the fuel warning lights on their vehicles come on well before the tank is actually anywhere near to empty."