For all who commented on MPG - MPGe is just a tool to allow comparison to a gas-only car. It will depend on what % you drive on electric-only vs gas-electric split.
All plug-in hybrids if this type have MPGe ratings (most overly optimistic) that are calculated per the same % of driving type. Larger all-electric range usually means larger MPGe, etc.
So, for the X5 with 19 miles all-electric range (AER) your MPGe depends on driving style very greatly. The X5 is a heavy vehicle, probably heavier with electric motor/battery/charging system. At least they had the good sense to combine it with the 2.0L turbo 4-cyl, which is already very light.
As for the i8 actual MPG from users, anyone who watched Top Gear's review of it saw the same - when you drive it aggressively cross-country, you are using it in the least electric-only way (even accounting for brake energy regeneration). It should be no surprise the MPG ends up low in its range.
If you have an i8 (congrats by the way ) I'd be interested in hearing about how you use the car (weekends/daily commute/city/suburbs etc).