We had a Lincoln Town Car with a digital readout of miles left in the tank. On one trip I miscalculated the distance to the nearest gas station and found myself on an empty stretch of highway with the numbers dropping ominously. I finally decided the only recourse was to head for a small hamlet off the road that might - or might not - have a gas station. The readout dropped to zero, then went out completely. We got another five miles or so and made it to a gas station. So you can get at least a few miles after the zero goes out. How many, someone else can check. Preferably with a spare gas can handy.
I have had the gas light go on a number of times in my Plymouth Voyager van and gotten 20 or more miles. Those give you a lot more grace than a digital readout.
By the way I concur totally with the folks who say running completely dry is a bad idea. It can be bad for the pump, you can suck who knows what garbage from the bottom of the tank, and it might be difficult to start again.
I drive a town car. Oh did I mention that it holds ww people. I have to confess to a terrible habit. When the tank gets low and that annoying chime comes on to tell you you have 50 miles left to empty, I look at my wife and say "look at that(as if completly surprised) we have 50 miles to go. I do ther same at 25 and 10. She wants to kill me and says I am going to run out and be sorry as you can't push a car this size. I can tell you from experience you can go exactly 32 miles after the "miles to empty" reads zero. Keep in mind it is a computerized calculation. Meaning the more you idle the engine the lower the number. The longer you drive the higher that number is. It is based on how many miles you drive vs mony many gallons of gas you have used