The amount of CO2 a car emits is directly related to the amount of fuel it consumes. For instance, a diesel car emitting 95g CO2 per kilometre consumes around 3.7 litres of fuel per 100km, while a petrol car consumes around 4 litres/100km for the same CO2 emissions.
Fuel efficiency, sometimes referred to as fuel economy, is the relationship between the distance travelled and the fuel consumed.