2003-2008 Mazda 6
It carries 18 gallons and averages 24mpg between city and highway.
So 378 miles city driving, 522 miles highway driving, 432 miles mixed driving.
When the gas light comes on, you have about 35 miles left before you run out of gas completely.
I know the older Mazda 6's do not tell you how many miles before the gas runs out, but that might be an optional feature on the newer Mazda 6's.
2009 Mazda 6
It gets 21mpg city and 30mpg highway with an 18.5 gallon tank, which means 388.5 miles on a tank of gas in city driving and 555 miles on 1 tank of gas if it is all highway driving. And about 470 miles in mixed driving.