Most electric cars are used for city commuting and do nowhere near 100 miles in a day, they're not appropriate or designed for inter-city or longer range driving.
In point of fact because at the moment the UK's electricity generation is mixed the actual emmissions from something like a GWhiz are not zero but are about 62gm/KM
That places it lower than any petrol car but given it's size it's not a fair comparrisom
adjust for that weight and it's not hugely better than many small petrol cars
What it does do is export pollution outside of cities where the build up of emmissions can be so damaging.
Electric cars are not a complete solution to our energy problems, just a part of the picture.
Longer term things like Hydrogen fuelled cars are probably a better bet but there are a lot of generation, storage and transportation isssues to deal with there so they're a way off yet