Petrol gives 19.64lbs of CO2 per gallon, diesel 22.38lbs. For example, my diesel Volvo V70 does at least 40mpg, so 40 miles gives 22.38lbs. However, a petrol model doing 35mpg will use 14 per cent more fuel, so for 40 miles will also give 22.38lbs. Why the crackdown on diesel, especially when you consider the diesel car engine will actually last longer and therefore cost less in emissions to replace?