At one point, when I actually had an owner's manual, I read that these car either have an 11 or 13. Pretty much it was the manual Pony's that had the 11, which was termed gas miser. Though my recent Pony part out had a 13 gallon tank.
You need to remember that a lot of liquid can fill the filler necks. And the fact that none of the cars I've had (have) read the same. There's always a big difference (2-3) in gallons when I fill up, even when the car is on e. Some run for a day on e, others spit and sputter just above e.
The instant reading thing is a joke anyway. You go from 99 mpg coasting down a hill to 1 tramping the gas up a hill. It should be the overall mpg that matter anyway. Which you need to the math when you fill up.