I have 2003 PT175 and the literature/web site says it is a 21 gallon, but I think it is a typo. I think it is actually a 12 gallon tank based on my fill-ups. I run a 25 hp 4stroke usually on a 600-1300 acre lake. I do not know for sure, but seems like it does not use much at all. I went to Lake of Egypt last fall and fished three days with three of us in the boat and still had plenty of gas left when I got home.
41 gallons; 150 h.p. carbed Mercury, and I use about 10 gallons in a hard day's worth of fishing depending on how far I have to go to get to a productive spot. On some days I don't burn more than a couple of gallons, on others I can run the tank half dry just being a speed demon.
Also 57 gallons, 225 Opti will will give me about 4.5 - 5 days average use (Not Tourney Use). The EFI 250XB will move the needle on the gas gauge almost as fast as the boat. The DFI 250XS lies in between the two on gas performance, with my overall preference going to the 225 Opti (DFI).