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Getting Better Gas Mileage In Warm Weather? Here's Why

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Fuel gauge

Fuel gauge

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You might have noticed, driving through the summer months, that you're filling up the car a little less often than you would over winter.

It's not your mind playing tricks on you, but a fairly common occurance for all drivers. Cars simply get better gas mileage during warm weather than they do when it's cold.

There are several reasons for that, and we've picked the most relevant ones below.

Warm air

There's a scientific explanation for why you use less fuel in warmer weather, and it's to do with air temperature.

As you may know, your engine uses air, as well as gas, in order to generate energy. In fact, it uses huge amounts of the stuff, though only the oxygen is used in combustion. These oxygen molecules combine with carbon in the fuel during combustion, to form--you guessed it, carbon dioxide.

Anyway, one property of air, like many substances, is that it expands when it warms. This makes it less dense--molecules of the various gases are spread further apart. This means that every gulp of air your car is taking during combustion has less oxygen in it at warmer temperatures, and if there's less oxygen, the engine compensates by using less fuel.

This is bad from a power perspective, but good for economy. The engine is combusting less fuel due to the warm air, improving fuel economy. During winter, the reverse is true--denser air encourages the engine to chuck in more fuel, boosting performance and harming economy.

2012 Audi A6 all-LED headlights

2012 Audi A6 all-LED headlights

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Accessories

If you live in a particularly warm climate, then you probably use your air conditoning or climate control most, if not all the time.

This naturally uses fuel, but the draw in colder weather is even greater, with heaters constantly on, more use of headlights in the longer winter nights, and even spells on the driveway with the engine idling, simply to defrost the car.

By comparison, you'll use less gas in warmer weather simply from not running as many accessories. All those accessories have to get their power from somewhere, and that somewhere is usually the alternator, which draws engine power as it spins. Fewer accessories mean less gas.

Weather

We've already covered air density, but the weather itself can affect your mileage too. It goes without saying that conditions are more likely to be adverse in winter--particularly in some areas, where heavy snowfall or rain is more likely.

Racing Green Endurance - A rainy Start

Racing Green Endurance - A rainy Start

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When the road has less grip, you'll likely use more energy simply moving than you would on a bone-dry road. Every spin of a tire is wasted energy, and therefore wasted fuel. And water on the road increases rolling resistance. That all means that those summer drives end up being more efficient.

Then there's traffic. Although you might get caught in long queues during vacation times, poor weather conditions do slow down traffic and result in queues. With fewer queues in summer, you save a little more gas than you would in winter.

If you live in a state with particularly defined climates, then you may well use dedicated summer and winter tires, too. Winter tires are great for improving grip and safety in adverse weather, but the softer rubber compounds used in winter tires increase rolling resistance. Those summer tires slip over the road surface more easily--again, improving gas mileage in clement weather.

Other factors

There are other factors increasing rolling resistance too. One of those is air pressure. Though we've mentioned how warm weather means lower pressure outside, in your tires--where the air cannot escape--that increase in temperature results in higher pressures.


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Comments (25)
  1. I have always wanted to know the answer to this question. Unfortunately the author supplies no data or links to support his prose.

    The initial claim of air density seems totally off the mark. The engine management system will adjust to put out the needed power so efficiency should not change...however...

    When it is hot out side, the air is less dense and easier to push out of the way, and therefore the car is more efficient.

    So perhaps the author has the right parameter, but the wrong explanation. Does anyone have any real data?

    And the idea that the heater draws more power than A/C seems to neglect the fact that heat is "free" for ICE cars.
     
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  2. Yup, missed that one too: aerodynamic drag is probably the biggest single factor.
     
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  3. huh?

    Some of MOST unscientific explaination ever... Terrible article.

    First of all, huge assumptions about the weather pattern in the winter vs. summer. In states such as FL and AZ, or most Southern states, it is actually rains more in the summer than winter. Maybe it is the case in England, but it certainly doesn't apply everywhere...

    2. Sure, hotter air is less dense. Less air equal to less fuel. But where is the proof that "air flow sensor" will detect a difference in air temperature adjusted flow rate? All cars are computer controlled that its fuel will be adjusted based on driver input and air intake. If the car lacks power, the driver might drives it slightly harder to compensate for the difference and increase fuel use.
     
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  4. You observation that it rains more in summer than in winter is partly correct. This is also true for Western Europe, although the difference (measured in mm's) is small.

    What does make a difference is that rain in winter usually falls in short bursts and due to the higher temperatures, the road surface dries up quicker.

    In winter, the road can stay wet for days on end. Not because of more rain, but a lack of evaporation.
     
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  5. Oops, I meant to say that the rain in summer falls in short bursts.
     
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  6. Continued,

    Accssories arguement are okay. Assuming that you have longer day lights during the summer over winter so people don't have to use headlights as much. That can impact your mpg. Heaters are essentially free for ICE cars (non-hybrid). The colder climate might require more electric defrosting and longer "warm up" (bad practice) time so drivers will burn more gas that way.

    Lower temperature will result in lower tire pressure (but any good owners will adjust that monthly). The oil argument is silly b/c it only impacts "start up" temperature. Once the engine is warmed up (fairly quickly), the temperature difference is miniscule. There are auto shops that switch different grade of oil between winter and summer.
     
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  7. oil temperature is usually not controled by a thermostat (like coolant water). so it is cooler in winter and has increased friction -> lower mpg -> close the radiator grill (at least alittle bit). or use oil with lower SAE nr.
    "warm up" without driving the vehicle is just for idiots. time the engine is running in cold and dangerous conditions is increased by this stupid manner. such people usually can not read or even calculate mpg;-)
     
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  8. Continued,

    as far as air resistance go, that is only a hwy assumption. If you drive slower, the difference are undectable.

    Mostly, all those differences can be easily masked by how heavy your foot is.

    Out of all the factors, the tire argument is valid if the owners don't check their pressure between season. The oil argument is ONLY valid if shops change the grade of oil between winter and summer. Most of the gas wasted in the winter are due to excessive unnecessary warm ups by the driver. Headlight and electric defroster contributes some, but they are all small "potato" in the large scheme of things...
     
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  9. Thanks for the added input.

    The fact remains, I get significantly lower gas mileage in the winter than in the summer on both my ICE car (Corolla) and hybrid (Prius).

    I watch my tire pressure and don't do anything different in warm up (get in and drive). I am not sure what accounts for the difference, winter to summer.
     
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  10. Well, I expect EV and Hybrid to get worse MPG b/c battery loses charges in colder temperature. Your Corolla probably gets worse MPG due to the cold start where the initial few miles, the engine and emission system will take longer to reach "efficient" states. The longer it takes, the less efficient it gets. That is probably the "main" contributor.
     
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  11. yep!
    cold start (like choke in old times)still happens even if you do not "idiot-warm up" the engine and start driving immediately.
    so what is the everage distance you drive with car, starting in cold engine conditions?
    if this distance is low this "choke" has high percentage and you see reduced mpg-
     
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  12. Another factor for those using all weather tyres: warm tyres supposedly have lower rolling resistance.

    Plus hybrid behaviour. The Prius does a warmup cycle when you start it. It does this afaik for two reasons: 1 warm up the catalytic converter 2 warm up the interior. Only after this has occurred, will the full hybrid mode kick in. This makes the Prius extra sensitive to cold weather. You loose the hybrid advantage for the first few km.

    One trick I learned is to switch off the heater in winter during the second and third km. After the 3rd km, I can switch it on at the lowest setting (16 degrees C), without dropping out of hybrid mode.
     
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  13. The FOURTH post in this forum has an interesting graph. Clearly Air Temperature (and presumably density) is important.
    http://ecomodder.com/forum/showthread.php/outside-air-temperature-mpg-14440.html
     
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  14. Well, it is commonly known that colder air is denser. Denser air will produce higher compression and more power...
     
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  15. Is it that, or the higher drag created by denser air.
     
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  16. Do you "gain" efficiency driving at higher elevation due to "thinner" air? Well, your engine lose efficiency faster than your car gain in less air resistance...
     
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  17. Maybe we should compare the MPG rating the same car in Denver vs. Florida and see if there are any difference in effiicency gain... :)
     
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  18. you guys are picking nits. the fact is, the story is not based on any facts or valid science. It's based on the gut feelings of a moron.
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  19. @Rick Roberts: This is your friendly site moderator. Keep it polite, please. Criticize on the facts all you like, but no insulting authors or other commenters, OK?
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  20. yes but only if throttle is open completely.
     
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  21. This has been fun... [giggle giggle]

    Peace
     
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  22. One important point overlooked here is that the fuel itself is warmed up considerably during the summer months and therefore the the fuel atomizes easier than when its cold - you can actually buy kits to prewarm the fuel using the engines coolant! although its more popular on diesels rather than gas engines but many claim a 5-10% mpg increase using this device.
     
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  23. well do serious racing teams use such "devices"????
    this is all compensated by ECU
     
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  24. Poorly written article. Not only does it spit in the face of the laws of thermodynamics, especially as they apply to IC engines, but it also shows an utter lack of under hood experience. The absolute absence of any supporting data outside of "personal experience" tells a great deal as well.
     
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  25. your statement "...and if there's less oxygen, the engine compensates by using less fuel...."
    is not correct! if the engine uses less air and less fuel it provides les power right? and thos is compensated by the driver. noone is aware abought the exact pedal position!

    tyre pressure. you are right. just check when it gets cold! mpg isthe same.

    winter tires: not correct. winter tires have much more tire profile. so the same tiresize has more diameter compared to the summer tire. more diameter means less rotations for the same distance. as trip is calculated by counting rotations you will see less miles on wintertires compared to the same track by summer tires...so you see less mpg. don´t belief in everything you see!
     
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