Range extender option
An optional petrol/gasoline range extender engine is marketed as REx and is powered by the same Kymco built 647 cc inline two-cylinder engine used in the BMW C650 GT maxi-scooter. The system is intended as an emergency backup to extend range to the next recharging location, and not for long-distance travel.
 The 647 cc engine as fitted in the i3 REx is derated compared to the engine of the same displacement in the C650 GT; the REx engine develops 34 hp; 34 PS (25 kW) and 41 lb⋅ft; 5.6 kg⋅m (55 N⋅m) at 4,300 RPM, achieving compliance with Euro-6 emissions standards compared to the 59 hp; 60 PS (44 kW) (at 7,500 RPM) and 49 lb⋅ft; 6.7 kg⋅m (66 N⋅m) (at 6,000 RPM) for the engine in the C650 GT, which complies with the less restrictive Euro-4 emissions.
The REx engine operates when battery capacity drops to a pre-specified level, generating electricity to extend the range to about 320 km (200 mi) for the European version.
European models have a 9 litres fuel tank while American models have the same tank though software-limited to a capacity of 7.2 litres. Under EPA five-cycle testing, the i3 REx has a total range of 240 km (150 mi).
The vehicle's official range is 130 to 160 km (80 to 100 mi) for the 60 Ah battery option under the New European Driving Cycle (NEDC), and up to 200 km (120 mi) in the most efficient driving mode. Under the United States Environmental Protection Agency (EPA) cycle the official range is 130 km (81 mi) with combined fuel economy of 124 miles per gallon gasoline equivalent —MPGe— (1.90 L/100 km; 149 mpg imp).
In July 2016, BMW released the 2017 model year (MY) BMW i3 94 Ah with an improved 33 kWh battery pack with 50% more capacity than the previous model resulting in an increase of its range to 183 km (114 mi) under the EPA cycle, and 300 km (190 mi) under the NEDC test. The Range Extender (REx) variant also has the same higher capacity battery as the all-electric model, with a corresponding all-electric range increase to 156 km (97 mi) under EPA testing cycle, for a total range of 290 km (180 mi).