There is also the hidden issue (always) -- the energy and emissions cost of producing the vehicle itself.  iirc the average US car has already been responsible for 30 percent of its lifetime's emissions burden (greenhouse gases etc) before it leaves the showroom floor.  The notion that we will fix all our problems by replacing the entire national vehicle fleet with clever hybrids seems to me fairly risible;  on the timescale necessary to do something effective about carbon emissions and climate destab, it would mean a car manufacturing spike (in emissions and energy/fuel consumption) that would kind of defeat the purpose.  Manufacturing the traction engines only and repowering existing (suitable candidate) vehicles would be more practical... but alas a huge chunk of the US fleet consists of carcasses so grotesquely large, anti-aerodynamic, and heavy that they are not, in fact, suitable candidates.

IMHO the new technologies for autos are desirable and practical only in concert with a serious transport mode change, i.e. car sharing, public transport, rail and bus expansion, curtailment of air transport, etc.   By themselves, cute new ICE technologies are merely bandaids on a gangrenous amputation -- or less colourfully, consoling toys for affluent people enabling us to kid ourselves that we are Doing Something About the Problem.  [Titanic, deck chairs, and so on.]  My $0.02...

The difference between theory and practise in practise ...