Last week I was on my way to the gas station with 2 carrots, when I made a sharp turn at moderate speed all of my gas must have slid to one side of the gas tank and I lost power. It misfired and almost stalled in 4th gear, not good for the fuel pump either! On 4 carrots I'm refueling!

 

Well I put gas in tonight and my car took 14 gallons haha. Yeah, about that 13 gallon tank capacity.

 

Just looked at my fuel logs (only had the car 2 months) and I've put 13.2 and 12.3 as my top 2. The 13.2 I did let it run down to 1 carrot because I was in a hurry that day but the miles to empty started bottoming out on me (went 6 miles, computer dropped 20).
I think the Torque app connected to obd port might be a little more accurate. It gives me a percentage of gas left that seems to drop more steadily than the 200 miles before first carrot drops gas gauge.

 

Gasoline's thermal expansion is quite small, less than 1% for every ~20 degrees F. Not nearly enough to make up the 1+ gallons we are talking about.

There's also a very large difference between what gets measured in the lab at one specific time, and what gets printed in the thirty-second printing of a manual. Typos happen, and they like to propagate.

Since we have multiple people from different areas using different pumps at different stations who have put more than 14 gallons into the tank (and have driven in to the station to do so), I think we can pretty definitely say that there are some MINI Coopers that have a fuel capacity of over 14 gallons.

Unless you've actually measured your tank's capacity, all you can say for sure is "it holds more than" whatever the most you've put in.