Hi Ken,

You haven't specified a particular car so this is a bit of a stab in the dark. Mostly it depends on the age - more modern automatics have hit a peak of efficiency and so there is generally now either no difference to speak of or even sometimes a slight fuel economy and CO2 advantage for the auto. On older cars the difference is starker - fuel consumption can be as much as 10- to 20 per cent worse with the auto.