the tank "holds" 18 gals....
Yes Drew, that is why I first thought I had gotten a cheap repro tank in , I knew former owner put in a new tank...and I knew the tank was supposed to hold 18 gal, but I ran out twice while my gauge wasnt working , and both times I could only fill in 15 , and just over 15 gals.....so I thought I had a size wrong tank, ordered a new , installed and to my surprise found it was the same dimensions , and can only fill up w 15 -15 plus if "empty"...I hear talk about 'air bubble', but also that pick up tube may not be all the way down...?

I wouldnt count on getting 18 gals worth of driving out of it even if you put in 18 gals after tank has been out...
You can test your own 'capacity' by driving it to empty (just have a couple of gals w you in some container!), then filling it up...