I'll safely say 1.5 gallons but it might be 2 gallons. I've gon over 40 miles once with the gas light on ans I think there was still 1/2 gallon or so left but the pickup tube dosen't touch the bottom of the tank, or at least it shouldn't.
Ive pushed it pretty good on mine and I ended up going about 50 miles, but that was on the highway too. I still had a little left and I wouldnt push it that far in the city. The Focus uses a different type of pump than does my current car.
My current vehicle uses a venturi type pump and is EXTREMELY good at scavenging fuel off the bottom of the tank. The Focus fuel pump uses a different style of pump and I wouldnt recommend running it dry.
____________________ My car is a synner! It uses Amsoil Synthetic Lubricants.
2001 Pontiac Grand Prix GTP
It whines a lot, but I like it
I've pushed the be-jeezus out of my empty light and have been shocked to find that I could fill my tank on my 2k3 PZEV with 13.8 gallons - That is the most I've ever put in any focus I've owned or had the pleasure to drive and fill.
I would give yourself a gallon + maybe a half, but this here is subjective.
____________________ Get in, Sit down, Shut up and HANG ON!