I think the argument (not sure how true it is because I don't know the facts) is that we can do more to (in the future) reduce carbon emissions from power plants and that we will get benefits of scale when we move burning fossil fuels to bigger plants as opposed to doing so in every car.

That's a horrible talking point that gets repeated by the people who have never done the math. Burning that same fuel in a modern efficient (80%+) turbine and then using the resulting electricity in an electric motor (also 88%+ efficient) is way way better than burning the fuel in a combustion engine, which is like 20-30% efficient at best.

Isn't electricity's transmission efficiency less than 50%?

More like 5-6%. A quick googling yielded: https://www.eia.gov/tools/faqs/faq.cfm?id=105&t=3

Charging your battery might be less efficient.

They did the whole math in "Sustainable energy without the hot air", and over all, even burning coal and using an electric car is better than burning petrol in a comparitively small engine.

Funny enough, a similar analysis applies for heating: burning natural gas to make electricity to drive a modern heat pump is more efficient than burning the gas directly for heat at the premises.

From a physical point of view, that's possible because you only need low grade heat for your home, but burning gas produces high grade heat.

Just to clarify, the efficiency is 100% minus 5-6, or ~95%. (I know that's what you meant, just in case anyone is confused.)

Thanks for the clarification, I read it backwards first and did a double take.

Transmission losses in the US are 5%.

https://www.eia.gov/tools/faqs/faq.cfm?id=105&t=3

actually gas to electric efficiency is max 50% (60% if co-generating heat)

source: www.eurelectric.org/Download/Download.aspx?DocumentID=13549

"When waste heat is captured from these systems for heating or industrial purposes, the overall energy cycle efficiency could approach 80 percent."

http://energy.gov/fe/how-gas-turbine-power-plants-work

The trouble is that you're comparing apples and oranges, or rather efficiency of electricity generation with efficiency of heat generation. Remember that thanks to heat pumps, electric heating can often have greater than 100% efficiency, so waste heat is less useful than the same amount of electrical energy even for heating.

I think you don't understand what co-generating heat is. It's a part of generating electricity. You use that heat to heat up the next batch of water, so you end up spending less fuel turning it into steam.

https://en.wikipedia.org/wiki/Cogeneration