Yeah, I assumed 50% to account for various losses and other less efficient generators. I don't know what the actual number is - probably in the 40-50% range.

Interestingly thanks to renewables the "carbon intensity" of a kWh of electricity is already better than a kWh of fossil fuels in some countries. A kWh of petrol releases about 250g of CO2, and quite a few countries (predominantly developed ones) have lower carbon intensities per kWh of electricity. Considering round trip efficiencies the carbon released by an electric car in those countries is easily 1/3 to 1/4 of the carbon released by a petrol car.

In the USA it's not as good, but electric cars are still releasing about half the carbon.

The big question in my mind for efficiency figures is whether it's with respect to the Lower Heating Value (LHV) or Higher Heating Value (HHV) of the fuel. The latter includes the latent energy from condensing the water of combustion.