My handbook says that when the light goes on there is 2 gallons left. It comes on roughly when there is an 1/8th of a tank left. But i've never driven more than 20 miles after the light has come on because;
a: I have a diesel so if i run out i can't just refill it, i need to call someone out to pump the fuel through.
b: i dont trust there is 100 miles left, the needle seems to go down too quickly.
c: unless you regularly run your car very low, gunk and debris from old fuel and fuel imperfections can accumulate which can block filters quicker (dunno if this is a myth, as surely the fuel pickup has to be at the bottom of the tank anyway?
I know this doesn't
help too much, but i assume the quantity a 1.6 has when the light goes on is the same, but being petrol you can't go quite as far. Unless you live in the arse end of nowhere, you normally have enough to get to the nearest petrol station, just can't be too picky about which one you stop at.
Bear in mind it is an offence to travel on the motorway if you know you are running out of fuel ( but dunno how often they prosecute on this)