Since you started publishing figures for sub-100gm/km CO₂ petrol cars in your Data Files, I have spotted an apparent anomaly between these figures and the fuel economy in mpg. I’m pretty sure that your Diesel Car figures must be correct, so can you explain why a Golf 1.0 TSI 114bhp petrol had CO₂ emissions of 99g/km and fuel economy of 65.7mpg, when a Golf 1.6 TDI 109bhp with 99g/km CO₂ has a fuel consumption of 74.3mpg? I thought that fuel economy and carbon dioxide emissions were closely interlinked, with more fuel used and more CO₂ produced in proportion. 

Ted Strike

Sure can Ted, although I won’t dive too deeply into the chemistry and physics of the two fuels. Of course the 1.6 TDI uses less fuel than the 1.0 TSI because diesel engines are more efficient. But there’s more to it than that, because there’s more energy in a gallon of diesel than a gallon of petrol ñ around 15 per cent more! It’s because there’s more carbon and less hydrogen in diesel fuel, making it denser, and you get more carbon dioxide from diesel because a litre of diesel weighs around 850 grams, and a litre of petrol around 740 grams. This all adds up to an overall economy bonus of around 25 per cent for diesel engines, but if we measured fuel economy by miles per pound weight, or take price into account (which is partly why diesel costs more per litre,) it doesn’t look quite so good. As a cross-check, if you multiply the mpg by the g/km CO₂ for any car, you will always get a figure of around 7,500 for any diesel car, and around 6,500 for any petrol car. I might add that, when a few per cent of green ethanol is added to petrol, as now required by law, the energy content per litre drops noticeably, as ethanol has 35 per cent less energy per unit weight than petrol, which affects the fuel economy. Blending in biodiesel to normal diesel, as is also required by law doesn’t make such a big difference, as it contains only 10 per cent less energy than mineral diesel.

Hope this sorts it for you.

Doc