I've never underwstood the cold temperature mileage hit, can anyone enlighten me?
With conventional, non-hybrid ICE cars, lower temperatures generally increase engine efficiency, at least down to temperatures lower than 50 F. One reason is that colder air is denser, and thus more oxygen is sucked into the combustion chamber at lower temperatures. That effect of course is much more pronounced with forced induction engines like my former supercharger MINI Cooper S, which - like most FI ICEs - had an intercooler. On a dyno, you can clearly show the effect of the intercooler. Of course, without the compression, there is less of an effect.
All that aside, why is a hybrdi drivetrain less efficient at colder temps, when the ICE itself should be more efficient at colder temps. Is it merely the longer warm-up time?
For that, the difference between 75 ad 50 F should be negligible.
I remain puzzled.