Recent developments reveal that the fuel tank capacity of the i3 REx was not physically reduced, but rather controlled via software. This was accomplished by shutting off the fuel pump at 1.9 gallons.
More details on BMW's i3; electric and connected - Green Car Congress
This Is Why You Should Buy A 2018 BMW i3s
BMW i3 REx: Road-Tripping With 72 Electric Miles, 1.9-Gallon Gas Tank
Is the BMW i3 'the most advanced vehicle on the planet?', Page 3
bioage.typepad.com/.a/6a00d8341c4fbe53ef01901e32a0
Review: Why I'm returning my BMW i3 after three months
2019 BMW i3 Road Test Report - Consumer Reports
How to Hack a BMW i3 For More Driving Range
Why the BMW i3 has such a tiny gas tank - Autoblog
Gasoline range extenders like those in the BMW i3 mean more EV miles travelled
Is it possible to drive a BMW i3 REx (range extender) with gasoline only, and not to charge it at all? - Quora
RE: BMW i3 PH Used Buying Guide - Page 1 - General Gassing - PistonHeads UK
BMW i3 Review (Page 2) - autoevolution
So here is what a BMW i3 electric Motor Looks Like (removed due to a Power Electronics Recall) : r/Justrolledintotheshop
Pre-Owned 2016 BMW i3 with Range Extender in San Antonio TX