getting very low mpg...

Hi all,

I just bought a 2000 cabrio GLS with 80,000 miles. I have just been driving it for a few days but the gas mileage seems to be much lower than expected. I have been driving mostly on the freeway and so far I have done 80 miles with about one third of the tank. That would put the full tank at 240 miles. I read somewhere that the tank was 13.7 gallons. That would mean an average mpg of 17.5mpg!!! I was expecting somewhere around 24mpg..
any idea what the problem could be?

thanks,

franck-

check and see if the spark plugs and fuel filter and air filter have been replaced (they are all part of the 80k mile service which is usually when people trade the car in because its about 5-600.00)