Modifying your car outside of factory specifications isn’t always risky, depending on what you do to it, but this particular i3 enhancement certainly is. It doesn’t necessarily void the warranty, but putting several gallons of fuel in the middle of a car’s crumple zone is risky.
These areas are intentionally designed to collapse during a collision in order to disperse the force of the impact and better protect occupants from severe injury. If one were to put a gas tank in that trunk space (located in the front of the vehicle on the i3), it would likely rupture in an accident. Having gallons of gasoline spray everywhere after an impact creates a very dangerous scenario in the aftermath.
Even if you could somehow assure that your modded i3 would never be involved in an accident (virtually impossible due to outside factors while driving), there’s also the risk of performing the modification incorrectly. If enough care isn’t taken, if the connections aren’t entirely precise, if the tank itself is a cheap plastic affair or has any kind of structural issues, you could end up with gas fumes leaking into the car’s front trunk space. These are volatile, combustible, toxic gas fumes.
