Autocar’s Greg Kable didn’t say on what he is basing his “124 miles” figure, but he compared it to the current range of the i3 which he said is “just under 100 miles”. This means he is working with the New European Driving Cycle (NEDC) which is not very accurate in predicting real world range. The i3 has a NEDC-rated range of “80 to 100 miles” and an EPA-rated range of 81 miles. Based on the same increase, the new battery pack should bring the EPA-rated range to just over 100 miles, which would be a comparable increase to the 2016 LEAF with 107 miles of range – up from 84 miles.
Interestingly, Autocar asserts that the vehicle’s new battery will achieve the range increase while holding the same amount of energy as the current battery pack: 22 kWh (18.7 kWh usable). Kable’s sources are saying that BMW managed to significantly increase the energy density of the pack.
I seriously doubt the German automaker could achieve such a range increase just by having a smaller and more energy dense pack. For comparison, Nissan’s 2016 LEAF achieves 107 miles with its new battery pack, but they increased the capacity from 24 kWh to 30 kWh.
I do believe Autocar’s sources when they are talking about up to 124 miles (NEDC and ~100 EPA) for the next generation battery pack, but I would be very surprised if BMW doesn’t increase not only the energy density, but the total energy capacity of the pack to achieve such an increase.