electric car efficiency = 1.0
central generating station efficiency = 0.6
total system efficiency = 1.0 x 0.6 = 0.6

So, neglecting transmission and other losses, electric cars might reduce energy consumption in transportation to 1/3rd of current levels. But that is only if all central generating stations burn petroleum (oil, diesel, gasoline, natural gas) which of course they won't. And the current petro-based transportation system doesn't rely on synthetic fuel. So it isn't really a valid comparison.

And even if energy consumption is reduced that much, it doesn't mean we won't have to build more infrastructure, more powerplants, more recharging stations. And the range and recharge convenience of current battery tech isn't very good compared to liquid fuels. Might see the tables turned in a few years though, with lithium air batteries and similar tech.