When you fill the tank, the automatic shut off will stop filling once the fuel reaches the top of the tank. I believe that the tank itself is in fact 23 Gal.

I found though, after the auto shut off, that if you very slowly fill the tank up and stop, and again, and stop, and again, etc. that the vehicle will take another 3 gallons of fuel and top off right to the top of the filler neck.
Be aware that this will take an extra 10 minutes.

As long as you're in no rush, and enjoy people watching like I do, the extra time is no bother, and plus, I leave the station with 26 gallons of fuel.