First off, I like the i3. As a city car (say London within the M25) it makes a lot of sense. However, having driven both REx and BEV models both on 24h test drives using same commute I have come to the conclusion that it is not quite as efficient as it might be.
Around town, at or below 40 mph, aerodynamics have little impact on a vehicle's range.
This post is about basic aerodynamic drag so I'm not including the complexities of interference drag (e.g: between mirrors and bodywork or turbulence created by wheels) nor surface friction concerns or Lift induced vortices.
We'll stick to good old Bernoulli's principles and Frontal Area.
BMW claim the i3 BEV has a Cd (non dimensional drag coefficient - basically describes how the car's shape affects drag) of 0.29 and Rex has 0.3Cd - doesn't sound too bad until you multiply that by the frontal area: CdA
This makes a lot of sense as now you can compare a huge variety of cars side by side.
OK so that makes the CdAs for the i3:
BEV: 0.690 m2
REX: 0.710 m2
Now compare this with other cars like:
1990s Honda Insight (2 seater hybrid): Cd = 0.25 but CdA = 0.474
1999 - 2005 Audi A2 1.2 TDI: Cd = 0.25, CdA = 0.544
1980s Toyota MR2 (2 seater) : CdA = 0.539
So the i3 is about as aerodynamic as a 1990s Toyota Camry or Subaru Impreza!
Even the 1995 BMW M3 is better: CdA = 0.629 m2 and possibly my least favorite car (the 2004 Prius) is 0.580 m2
The best examples in the noughties seem to be the VW XL1 (2 seater) with CdA of 0.279 and the German Loremo prototype (2+2): CdA = 0.250 m2
Sadly the XL1 is no People's Car and might better be re-badged the EB XL1 (a cheap £100k Bugatti !) Sadly Loremo still seems to be in the doldrums.