depends on how you drive. i noticed that the few times my light has come on there were still about 3-3.5 gallons left in the tank. but i never see the light because i hate running under a 1/4 tank of gas. i look for a place to fill up when i hit the 1/2 mark.
I get a range from 2-4.....I've never put more than 12 gallons in. And it does change from the way you drive, running like hell made me fill up 10, slow casual made me put in 12 gallons. Though, I still think the gauge is unreliable.
i dont know. I never like to go below a 1/4 of a tank. besides that I live in the seattle area with a lot of hills too, the needle on the gauge always moves. Generally like to say above a 1/4 though. Never tried the limit.
Look in the book that came w/ the car.
I was reading it the other day, and I THINK it said you have < 2.5 gallons left when the light comes on.
That would give me up to 70 miles (@ my highest MPG), though I could probably do better if I had to. My old K5 got 11 MPG on the highway, and had no fuel light. I pushed it one day and ended up running out of gas on the off ramp during rush hour. The Focus doesn't get the best MPG around, but is amazing compared to that thing.