I think you're missing the point about the motor. The BMW i3 uses an electric motor for propulsion, but that doesn't mean it's suddenly 80% efficient if the electricity is coming from the internal backup combustion engine. If anything, you are now only 0.2 * 0.8 = 16% efficient [1].

What I'm interested in is the efficiency of the whole process in the car that turns the fuel into motion. In the Tesla, the electricity is stored in batteries which have few losses and goes directly into a highly efficient electric motor, so it gets that 80% overall.

1: In practice, the setup electric engine for propulsion + combustion engine for electricity tends to be somewhat more efficient than just the combustion engine for propulsion, as you can run the combustion engine at a constant, efficient range of RPM.

I think you're focused on the motor, when that's the least important part of the equation. As you point out with the BMW i3, the source of the electricity is important. In battery mode, it is actually more efficient (27 kW-hrs/100 mi) than the Model S (35 kW-hrs/100 mi), but that's obviously not true when the range extender kicks in.

> I think you're focused on the motor

Did you read your parent post? "What I'm interested in is the efficiency of the whole process"