How winter weather affects your fuel economy

It's not just our bodies that struggle in the bitter cold. Our cars also find low temperatures tough. U.S. Environmental Protection Agency (EPA) tests

 show that a petrol-powered vehicle uses approximately 15 per cent more fuel at –7ºC than it does at 25ºC – with the penalty even greater on short trips. For hybrids and electric vehicles (EVs), the penalty can be even greater. But why? Here's how cold weather makes your car work harder – and what you can do to minimise the loss.

Because cold air is denser than warm air, cars don't slip as easily through it, meaning that the aerodynamic resistance needed to drive through the denser air requires more power (i.e. fuel) to compensate. The loss can be compounded by higher average winter wind speeds – in Victoria, the difference can be as much as 15 to 20 per cent. What can you do about these natural phenomenon? Not much. But if you leave your roof racks or roof box on when you're not using them – thereby increasing aerodynamic drag – you'll be copping an even bigger fuel-economy hit in winter. Best to take them off!

Just like honey in the pantry, the viscosity (or thickness) of an automotive lubricant or fluid changes with the temperature. The lower the temperature, the more viscous it tends to be, meaning more friction. The more friction an engine and drivetrain has to overcome, the more fuel is used. On a short trip, it might never actually achieve proper operating temperatures, meaning the whole trip is driven at sub-optimal efficiency. Parking somewhere warmer is one way to combat the discrepency, so if you have a garage, use it. Or, if you have multiple short trips on the agenda, combine them to allow your car to properly warm up. If your mechanic gives it the thumbs-up, you could also use specific winter-graded engine oil.

The reality of battery chemistry means they don't perform as well at the extremes of temperature ranges: especially in the cold. In a petrol-powered car, it means the alternator has to work harder to charge the battery, increasing fuel use. In an EV, both the batteries and the regenerative-braking system lose efficiency, decreasing your driving range.

All the features that make a car's cabin cosy and keep the windows clear – heaters, defrosters, seat and steering-wheel heaters – use a lot of power and make alternators work harder, impacting fuel economy. For that reason, you should use your warming gizmos only when you really need them. The penalty is particularly significant for an EV. According to the US EPA

, an EV with the heater on is 39 per cent less economical in cold weather compared to just eight per cent when it's off. Using seat warmers can be a less energy-sapping way of staying warm in an EV or you could consider pre-heating it when it's on charge.

Cool air is denser than warm air, which means your tyres lose pressure when they are cold. Because rolling resistance increases as pressures drop, your car has to work harder to maintain momentum. For this reason, you should always check your tyre pressure regularly and ensure they are inflated to specification.

Cold weather is often accompanied by wet, slick roads, reducing the traction a car has at its disposal and reducing how much of its power is translated into momentum. If you own an all-wheel-drive or four-wheel-drive and use the feature when the roads are really slippery, you're also increasing drivetrain friction and, therefore, fuel use. The good news, however, is the fundamentals of safe driving are equally beneficial for fuel economy. If you slow down, keep back from the car ahead, and stay smooth with your throttle, braking, and steering inputs, you won't just be maximising your odds of staying safe on the road, you'll be making the most of your car's precious energy source.

^Disclaimer

^{Viva Energy Australia Pty Ltd (“Viva Energy”) has compiled the above article for your general information and to use as a general reference. Whilst all reasonable care has been taken by Viva Energy in compiling this article, Viva Energy does not warrant or represent that the information in the article is free from errors or omissions or is suitable for your intended use.}