By the time that you’re reading this, it will be nearing mid-winter, officially the 21st December, and the days will start getting longer. Hurrah! But it doesn’t tell half the story. As we well know, the worst of the winter weather inevitably comes after Christmas, in January and February. Taking Heathrow as an example, in December it enjoys the least sunshine of any month of the year, but February delivers the lowest daily minimum temperatures, and has the most days of air frost. Your engine and the car windows and cabin take longer to warm up, any traffic hold-ups caused by road conditions do no good for fuel economy, along with the road conditions brought by frost, snow, fog and ice. You, your car, and its engine have a tough time in these sort of conditions, and it’s hard to focus on fuel economy when you’re glad enough to get home safe and warm, and without incident.

But also working against fuel economy are other factors. Around now, as I write in mid-October, the oil refineries and regional fuel distribution depots are thinking of switching over to deliveries of winter grade diesel fuel. This usually starts in early November, when fuel is blended somewhat differently, to help fight the problems of diesel fuel’s propensity to ìgellingî in particularly cold conditions. All diesel contains a proportion of heavier hydrocarbons that begin to solidify into semi-solid waxes at temperatures below minus 5 degrees Celsius, a temperature which is easily attained in many areas of Britain. There are two ways of lowering the CFPP, or Cold Filter Plugging Point, at which problems start to occur, often with blockage of fuel delivery at the fuel filter, or elsewhere where the fuel pipes are most exposed to the cold. In Britain, the official requirement is for the CFPP to be lowered to -15 degrees Celsius for the winter months. Often the refinery distillation cut is altered, to produce a thinner diesel fuel that’s less prone to gelling; alternatively, or additionally, they may blend in additives that delay the crystallisation of the wax. It depends much on the nature of the crude oil being refined as to which of these two ways (possibly both) is used by any fuel supplier.

What, you might ask, has this to do with fuel economy? Well, the energy content of this thinner winter grade diesel tends to be somewhat lower than that of summer grade fuel, and sometimes also has a lower Cetane value. Primarily, it means that there’s less miles of motoring to be got out of every litre or gallon. Experience shows that a variety of engines respond differently, and we have had correspondence from some readers who find the fall-off of fuel economy quite significant and immediately identifiable, with their first or second tankful of winter grade fuel. Yet some engines appear to be rather less susceptible to the change. Whilst there’s no doubt that in winter your engine takes longer to warm up, and loses more heat all the time it is burning fuel, it’s not just this and the fuel that may cause a typical fall of the order of 3 to 5mpg during the winter months then. Like fuel economy in general, it’s much dependent on the type of motoring, and the length of your journeys, how noticeable the drop may be to zero and how closely you monitor your mpg figures. Don’t despair if you see a fall-off in fuel economy, but don’t lessen your efforts to keep your car warm and from extreme exposure to the coldest nights. It will help you get a quicker warm-up, and lessen the small chance that you have of encountering problems with fuel delivery in the coldest winter months. If your car is exposed to the extreme cold, you may be advised to use a product like STP Diesel Winter Treatment, which also contains a Cetane improver, to help out with cold weather starting.

Victor Harman