It did not have a full tank, I put 13 gals. of diesel in it yesterday and the needle was on the 1/1 (full tank) the whole time. I am not sure if it has the 17 or 21 gal. tank, but I called the previous owner and he told me to just put fuel in around every 300 miles. As the gauge is not working, I am not able to accurately calculate my mpg.