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I've been keeping track of the fuel my car, a Ford Fiesta Ecoboost 100 hp, uses since the day I got it. In the last three years I've owned it, I've noticed a large (>20%) difference in fuel consumption between driving in winter and driving in summer.

My driving patterns are identical in winter and summer, yet my fuel consumption is much higher in winter.

I understand that cars have slightly more power when cold, but is the difference that big?

Fuel consumption in liters/100 km:

Fuel economy

  • 2
    In my experience it is just that traffic is worse in winter. So you stop and start more often. – Chenmunka Oct 21 '16 at 12:15
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    I always assumed it was an air density thing. Cold air is more dense, requiring more fuel to maintain proper ratio. – cory Oct 21 '16 at 13:24
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    Not worth an answer -- but make sure you check your tire pressure. If you're a "set it and forget it" kind of driver, the temperature difference can cause large differences in pressure. Cold weather -> lower pressure -> increased rolling resistance. – tpg2114 Oct 21 '16 at 15:33
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    Is there a regular period of the year when you don't use the car? During summers? – Zaid Oct 21 '16 at 18:43
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    Does it snow where you live? I wonder if slippery conditions or fresh powder on the ground might have a negative impact on fuel economy. – Ajedi32 Oct 21 '16 at 18:44
27

Your fuel economy change is caused by the fact you're not using the same fuel. Fuel changes twice per year from summer blend to winter blend and back again causing a change in the Reid Vapor Pressure (RVP).

In summer, the hotter temperature evaporates liquids easier causing more pollution, so it is blended to lower the RVP. This blend is more costly to produce.

While in the winter time, the RVP can be much higher so additives such as butane are used. While this lowers the manufacturing cost, it also burns more quickly resulting in a hit on the MPG fuel economy.

  • 1
    +1 I was on a super mileage team and we saw 10-20% mileage differences between the two blends, which almost perfectly explains the OPs observations – Godric Seer Oct 21 '16 at 16:42
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    I think this is the correct answer. Factually verifiable and is a consistent condition among all drivers (whereas other factors may not be common among all, such as idling the car longer or more traffic). – justinm410 Oct 21 '16 at 16:46
  • "do to the fact" should be "due to the fact" – a CVn Oct 21 '16 at 17:20
  • @spicetraders The HNQ has been very, very good to you! – DucatiKiller Oct 28 '16 at 1:52
  • Do we have a valid source that shows that Belgium uses a different blend of petrol in the winter? – Steve Matthews Jan 6 '17 at 12:58
4

Your vehicle does not operate at maximum efficiency until the engine has reached its normal operating temperature. This is generally when the coolant temperature reaches about 200 degrees Fahrenheit. In the winter it takes the engine longer to reach that temperature simply due to the colder ambient temperature.

4

Another reason: Winter tires. Choose between hard tires (good fuel economy, REALLY bad on snow) or soft tires (worse fuel economy, good on snow).

  • I've never run winter tires (probably a bit too frugal and not that smart of me but that's another question all together). – Ives Oct 22 '16 at 10:17
  • Not necessarily - depends a lot where you live, too. In many areas you can mostly get away with all winter tires. But I love driving up into the mountains. And where I live, not having winter tires and then having an accident means a lot of bad things happen to your finances and driving license. – TomTom Oct 22 '16 at 10:19
3

In the winter the ambient air is much colder, making the air more dense. Denser air will contain more oxygen and so the car will inject more fuel in order to maintain the correct ratios. This effect could be exaggerated by the fact your car is turbocharged and that the inter-cooler will be working much more efficiently in the winter, further reducing temperatures of the compressed air before it reaches the engine. This is also why your car feels more powerful.

When a car is started cold it will run in 'open loop' status. This simply means that the engine is running on a rich fuel mixture as the oxygen sensor won't work properly until it's heated up enough. When it is heated up the engine will run in 'closed loop' status meaning it will use the oxygen sensor to regulate the fuel ratio. During the winter it may take longer for the engine to reach 'closed loop' status. You'll notice the effect of this more if you make short journeys and not so much longer journeys.

I wouldn't expect either of these to have an increase in fuel consumption by 20% though, so it could be a combination of reasons.

  • 1
    One thing to remember about the cooler air in winter being more dense ... while the ECU will compensate for it, you'll make more power because of it and therefor you don't need to use as much "go" pedal to achieve the same results. It becomes a wash. – Pᴀᴜʟsᴛᴇʀ2 Oct 21 '16 at 15:07
  • "Denser air will contain more oxygen and so the car will inject more fuel in order to maintain the correct ratios." — Why can't the engine maintain the correct ratios by sucking in less air instead? – Tanner Swett Oct 23 '16 at 6:47
2

Presumably in winter you'll have lots of additional systems switched on such as the heating, demisters, headlights, etc...

All of these things sap your fuel consumption.

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    And in summer I might have the air conditioner on. Doesn't the air conditioner use much, much more power than the heater fan and the headlights? – Ives Oct 21 '16 at 12:16
  • The most energy hungry system in your car is the heated screen and my understanding is that Fords usually have both a front and rear. – Steve Matthews Oct 21 '16 at 12:21
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    I'm fairly sure that heated screens consume far less energy than an A/C compressor. – Zaid Oct 21 '16 at 12:32
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    For a car with AC, this answer is plain wrong. In many countries you use headlights all year and all day long. Summer vs winter is basically AC vs heating. Yes, heated screen can be the most power hungry electrical system with up to 2.5kW, but mechanic AC can easily reach 4kW. Heated screen is used only for a short period of time when you start, after that free waste heat from the engine takes over, while AC runs all the time. – Agent_L Oct 21 '16 at 13:55
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    I don't think it's relevant but the car was purchased and is driven in Belgium. – Ives Oct 21 '16 at 14:21
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Not sure if it applies to you, but when I switch to winter tires, I see a pretty radical drop in fuel efficiency, i.e., from 500+ km per tank to 400 km per tank of fuel, maximum.

  • I've never run winter tires (probably not that smart of me but that's another question all together). – Ives Oct 22 '16 at 10:11
0

Perhaps the air density may be a variable in this. Upon looking for a source on this, I found:

Finally, a vehicles aerodynamic drag is proportional to air density. On a 70-degree-F day, the density of the air is 16 percent lower than on a day with temperatures around 0 degrees F.

... in a Scientific American article.

-1

I've got a 2015 Toyota Prius V, and with a fuel drop of almost 35% in the winter months. Could figure out the fuel lost until I did the research on different fuels. My fuel was just better than 51 miles per gallon, but left to see family up north mpg dropped to 29-35 mpg. Some of the problem was head winds going north and the following weekend again head winds. With a 20-30 MPH headwind and some gust as much as 40-45 mpg. Never though of a drop this much.

  • 1
    Welcome to the site. I'm not sure how this answers the question of "What causes" the difference. – Pᴀᴜʟsᴛᴇʀ2 Jan 1 '17 at 2:03

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