"If legislative force is required to reduced the world's car-fuel consumption, simply banning engines above 1.6 litres from domestic vehicles would be a big start"
It's a seriously bad idea for governments to start dictating technology like that.
It makes slightly more sense to legislate on outcomes (like the EU target of new passenger cars hitting 130gm CO2 per km by 2012). What makes even more sense is to provide financial "incentives" to meet environmental targets. That way you can get the market to work for you.