Winter fuel consumption: why your car uses more in the cold

The short answer

• Cold starts cost the most: the oil is thick and the engine takes several kilometres to reach its working temperature.
• In very cold weather, fuel use in town and on short trips can rise by 11–49%. On the motorway, with a warm engine, only a few per cent.
• Warming the engine up while idling saves nothing: you burn fuel without moving. Wait about 30 seconds and drive off gently.
• Combining errands into one trip and checking tyre pressures when the cold sets in help the most.

Why your car uses more fuel in the cold

fueleconomy.gov, the US Department of Energy’s fuel economy site, lists these causes. The first ones matter most:

• Cold oil and lubricants: thicker, so there’s more friction in the engine and gearbox.
• The engine takes longer to warm up, and it runs less efficiently while cold. On a short trip, almost the whole journey is like that.
• The heated rear window, demisters, heater fan and heated seats all use energy. To demist, many cars also switch on the air-conditioning compressor.
• Cold air is denser and slows the car more: at 0 °C (32°F) it’s 7.3% denser than at 20 °C (68°F). You notice it at motorway speeds.
• Tyres lose pressure in the cold. Tyres inflated to 2.3 bar (33 psi) at 20 °C drop to about 2.1 bar at 0 °C. More on this in tyre pressure and fuel consumption.

A study by Argonne National Laboratory (US) tested several cars at −7 °C and 22 °C: fuel use rose by 2–100% depending on the car and the drive cycle. The biggest increases came right after a cold start, and the gap narrowed once the engine reached its working temperature.

How cold it gets, and what it costs

The US figures are measured at −7 °C (20°F), which is unusual in most of Spain. According to Spain’s weather agency (AEMET), the average January low is -0.8 °C in Burgos, 2.7 °C in Madrid and 5.7 °C in Seville, and the average temperature 3.1 °C, 6.3 °C and 10.9 °C. What is common almost everywhere is starting the car in the morning with the engine at a few degrees, and that’s where most of the extra fuel goes. In mild winters, the increase will be closer to the bottom of the ranges than the top.

Mind the conversion: fueleconomy.gov talks about mpg. A 15% drop in mpg means 17.6% more litres per 100 km, which is why the table shows slightly higher percentages than the source.

Hybrids feel it more

According to fueleconomy.gov, under the same conditions a hybrid’s fuel economy drops by 30–34%. In the cold, the petrol engine starts more often to warm itself up and provide heat, and the battery performs worse and recovers less energy under braking. Hybrids still use less than an equivalent petrol car in town, but the gap narrows in winter. Where they save most: regular hybrids.

Tips

• Don’t warm the engine up while parked. Start it, wait about 30 seconds and drive off gently, without hard acceleration or high revs until it warms up.
• Combine errands into one trip: a warm engine uses far less than several cold starts. For trips of 1–2 km (about a mile), consider walking.
• If you can, park in a garage: the engine starts less cold.
• Check your tyre pressures when the cold sets in, always with the tyres cold, and set them to the manufacturer’s figure.
• Use the heated rear window and demisters only until the glass is clear. The heater uses the engine’s warmth, but it does little until the engine has warmed up.
• Take the roof bars and roof box off if you’re not using them: roof boxes and luggage.
• To work out the cost of a trip, use the calculator. If you measured your real mpg in summer, bear in mind it will be somewhat worse in winter: work out your real mpg.

Frequently asked questions