Well, we’re into 2020, and the days are getting longer, but the worst of the winter weather often hits in January and February, which are usually also the least sunny months of the year. February traditionally serves up the lowest daily minimum temperatures, with the most days of air frost. Engines take much longer to warm up, windows take ages to clear, and bad road conditions aren’t good news for fuel economy. It’s hard to focus on driving economically in such circumstances, and on some days you’re glad enough just to get to work and back, or survive any outing without incident.

But other factors can affect fuel economy directly, not least of which is the “winter grade” diesel fuel that usually arrives at our filling stations in early November. Summer grade diesel fuel contains a proportion of heavier hydrocarbons that can begin to turn into semi-solid waxes at temperatures below minus 5 degrees Celsius, which is easily attained in many areas of Britain. Winter grade diesel aims to prevent this problem, by lowering the “Cold Filter Plugging Point” (CFPP) at which these waxing problems start to occur, causing blockage of fuel delivery at the fuel filter, or where fuel pipes are most exposed to the cold. In Britain, the official requirement is that the CFPP is lowered to minus 15 degrees Celsius for the winter months, when the refinery distillation cut may be altered, to produce a thinner diesel fuel that is less prone to this gelling. Alternatively, additives may be blended in to delay the crystallisation of the wax – or sometimes both, depending on the nature of crude oil being refined, which varies a great deal more than we may often appreciate. But the significant factor is the energy content of this thinner winter grade diesel, which is lower than summer grade fuel, and it may also have a lower Cetane value, meaning it’s more difficult to ignite, particularly in low ambient temperatures. Less energy in the fuel means that there are less miles to be got out of every litre, or gallon and, if you monitor your fuel economy, the November change can often be spotted with the first tankful of the winter stuff. If your car is regularly exposed to extreme cold temperatures, you might be advised to use a product like STP Diesel Winter Treatment, which gives extra anti-waxing protection, and also contains a Cetane improver that assists in cold weather engine starting.

But lower ambient temperatures have their own effects, in terms of lower engine efficiency. In the winter your engine gets a lot colder overnight, and simply takes much longer to warm up next day. Regular winter journeys of under 15 miles means your engine may never get up to its best working temperature. Not only is the fuel economy affected, but the engine may gradually get contaminated with residues that are burnt off easily in summer months, and a good conventional fuel additive can help your engine stay fitter and help avoid starting problems. Even so, your fuel economy may see a fall of 10 to 15 per cent during the winter months, but how much depends on the type of motoring, and the length of your journeys. Don’t despair if you do see such a fall-off, which you can blame on the winter grade fuel, but don’t lessen any efforts to keep your car warm and protect it from exposure to the coldest nights. They will help you get a quicker morning warm-up and lessen the chance of encountering problems with fuel delivery. Best of all, garaging your car keeps the elements at bay, and avoids having to clear frozen windows – often whilst your engine is ticking over, but barely supplying any meaningful heat for defrosting. If you don’t have a warm garage, using windscreen and other window covers can help get you moving and ensure that the engine is warmed up properly. And don’t forget to put some good winter grade screenwash additive in your washer bottle, to help clear mild frosts and the remains of ice and snow, and to assist you in seeing where you’re going. There’s nothing worse on a biting cold day than running out of screenwash, or finding out that your pipes and washer system are frozen up!