size of gas tank

I have recently purchased a 99 amigo 2.2l motor and I am wondering the size of my gas tank. My gas gauge registers full when it is full but after about 50 miles it starts moving very fast and after about 150 miles it registers empty and the light comes on. I now it is propabably a bad sending unit but I can't get it fixed at the moment and I am tired of stopping and putting 5 or 6 gallons of gas in it at a time.

17.7 gallons.

use your trip odo and fill up every 225 and you'll be fine.

or replace the sending unit.