Jeep had a similar thing going in the 80's and 90's. 
The standard Jeep came with a 15 gal fuel tank. There was an optional 20 gal tank available.
It was actually the same tank. The 15 gal tank had an extra pipe that was part of the fuel fill tube that stuck down into the tank causing the gas pump to shut down 5 gallons early.
People could remove the extra pipe and gain 5 gallons!