Electric cars have an efficiency of about 70-80%, if you count the losses of electrical transmission, charging and the engine. A car engine < 40%, in mixed use about 20%. So even if you could generate fuel from electricity at close to 100% - which you probably can't - the electrical car is way more efficient. And of course, even with synthetic fuel, you have some level of pollution, as no combustion engine is perfect.
Except your electricity has to come from somewhere. If you are in the US, it will most likely be from some kind of hydrocarbon. You are just hiding the pollution, not removing it.
It is much easier to make those centralized power generation sources green than it is to make all of the cars on the road just a small margin greener. If you are in the US then those electricity has rapidly changed from dirty coal to cleaner gas and a growing percentage of renewable over the past decade.
You can make the centrals more efficient (to a point), but you are still burning hydrocarbons. And then you must apply the same electric efficiency as before. There are losses every time you convert the energy.
By the way, "a growing percentage of renewable over the past decade" is disingenuous. From ~9% renewable in 2002 to ~13% in 2013 is "a growing percentage", but not enough to make a difference in the discussion. https://en.wikipedia.org/wiki/Renewable_energy_in_the_United...
This is not green (electric) VS pollution (hydrocarbon). It's hydrocarbon -> electric -> car instead of hydrocarbon -> car. It's modifying a whole distribution network in the hope of achieving higher efficiency.
It's all the way up to 1/8 of all our power? That is definitely enough to make a difference, it means we're within quite minor scaling factors of having it take over.
