Correct, it's 1.9 gallons in
Correct, it's 1.9 gallons in the US instead of 2.3 gallons elsewhere. It's also worth noting that the review Chris Knapman wrote for the Telegraph, which prompted you to describe the performance of the range extender as "pitiful", is not consistent with my own experience as an owner of an i3 REx. Never once I felt that the performance was limited or inadequate. The REx appears to have several power modes, and as the energy needs increase, so does the rpm of the REx engine. While it could be described as clearly audible at higher rpm, it's not intrusive and does not sound cheap.
It gets the job done, and the driver can be assured of being able to get to the final destination no matter what. About the only scenario where an insufficient power situation could arise are hills with significant gradients, and with a fully depleted battery. In such a scenario, the vehicle might slow down to about 40 mph, depending on the grade and other power requirements.
While not ideal, the i3 has been conceived as a city car, and it can climb smaller hills and inclines of up to 400 feet easily on the battery reserve in REx mode. To sum it up, while the i3 range extender solution is not as powerful and versatile as what's found in the Volt, it's adequate for most situation. It is not correct to describe it as "pitiful".