97 Accent Gas Tank

I just bought a 97 Accent. The question I have is this....I know that they car only has a 11.9 gallon tank in it, but I went to fill it today it only took 5 1/2 gallons of gas (the gas guage showed 1/8 tank of gas left). Now to me logically it should take 3 gallons of gas for a 1/4 tank so the 5 1/2 gallons seems low to me but it filled til it started to come out. What could be causing this? Also when I fill it, the guage is very slow on moving to the full line. Could anybody also tell me what a good mpg would be for this car? Any help would be appreciated


Terry
84 Grand Am (170K and still running)
97 Accent (82K)