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Why cold weather gives you lower gas mileage

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With gas prices soaring you really need to watch every gallon of gas you use. I know I am kind of OCD about my MPG in my car. I used to keep track of it on my iPhone and track it over time. Since last year when I got a new car which now keeps track of it for me right in the on-board computer.  Though it is still more accurate to calculate MPG after every fill up. 

So like many people you probably notice you get worse gas mileage in the winter than in the summer. The biggest reason for this is the COLD AIR. The ideal operating outside air temperature for your best MPG’s will be from 75°-85°. Once you start dropping below say about 68° the mileage decreases pretty rapidly. The biggest impacts occur under 45°F . In fact depending on your make and model your mileage could drop by as much as 20-28% from summer warmth to winter cold. So why such a big difference? Here are some of the main reasons why.

start-a-car-in-cold-weather

 #1 More Idling:

Idling burns gas with no MPG’s so it’s the worst thing for mileage. In the winter many people warm their cars up which just wastes gas. Most cars only need about 30 secs of idling before they are ready to go. Cars also will idle at higher RPM’s to warm the engine up faster in cold temperatures versus warm, which also uses more gas. Another problem is people will leave their cars running more to keep warm, thus burning more gas as well.

#2 Lower Tire Pressure:

For every 10° drop in air temperature outside your tires will lose 1% in PSI or inflation. For every 1 PSI that your tires drop in pressure you will lose about 0.4% in Fuel economy. So if your tires are under inflated by just 3 psi your car would go from getting 22 mpg to 21.7 mpg. It adds up over a tank of gas.

#3 Lower Engine Temperatures:

Your ideal engine temperature is around 150°-195° depending on your make and model. Most modern cars with computerized management systems for the engine will order up more gas in the combustion camber to compensate. More fuel is added to the air/fuel mixture when the engine is cold. Parking in a garage or combing trips when the engine is warm will help reduce this.

#4 Higher Oil & Lubricate Viscosity:

When your oil and oil pan are cold your oil becomes sticky and takes more energy to pump through the engine. Transmission and differential fluids do the same thing. Using synthetic fluids can help reduce this problem. Along with the garage or combining trips tip.

#5 Higher Electrical Usage:

In colder temperatures:

  • You use your lights more because it’s darker earlier.
  • You use window defrosters more, seats heaters and mirror heaters.

#6 Weaker Gasoline Blends:

Gas doesn’t like to vaporize at colder temperatures so oil companies change the gasoline blend  differently for cold weather markets. These changes make gas work better in the winter and at colder temperatures but it unfortunately also decrease MPG’s.

#7 More Aerodynamic Drag:

When it’s cold you have more drag on your car as the air passes over it. Cold air is just denser than warm air. A vehicle’s aerodynamic drag is proportional to air density, and the density increases as temperature drops. For every 10°  drop in temperature, aerodynamic drag increases by 2%. This is mostly at highway speeds and long trips on the freeways.

#8 Icy, Wet or Snowy Roads:

Cars that have slippage on their tires burn more gas. Now we don’t always have to worry about this but it is a small factor. If you have been driving in cold wet or snowy weather this adds up over time. Plus if you have automatic AWD or 4×4  and it kicks in while on slick roads you are using more gas.

(Sources)

http://www.scientificamerican.com/article.cfm?id=why-is-the-fuel-economy-o

http://www.fueleconomy.gov/feg/factors.shtml

http://www.metrompg.com/posts/winter-mpg.htm

 

  • Michael Flanik

    thanks for the explanation. I definitely notice less gas mileage during the winter months and I just kept blaming it on the tire inflation. Didn’t realize all these other factors.

  • http://twitter.com/cdiman7 Karl Sefcik

    Thanks for the article, though you still need to figure out mpg the way you used to. I noticed this once with my first hybrid. Car mpg displays are not true mpg, they are instantaneous mpg. What I mean is once my 2000 Honda Insight registered 150 mpg on that display. I had just filled up, and was going slightly downhill. It occurred to me then that the dashboard mpg displays the correct mileage ONLY IF you drive the whole tank (in my car then, it would’ve been close 700 miles), with the same road conditions (surface, angle), weather, tire condition. So no way to get out of long arithimetic, what I do is one trip set records the mileage after each fill-up, and I divide that by the gallons in the fill-up. Usually, the math gave me a higher mpg than the dash did.

    • wxbrad

      Karl, good idea I actually started doing that recently to help eliminate causes for my lower MPG during the winter.

  • http://twitter.com/cshotton Chuck Shotton

    The number of fuel molecules per cubic volume is much higher in cold weather than warm, as is the oxygen density in air. Are you sure you aren’t just making up the “worse fuel economy in winter” stat? As a pilot, we’re taught that engine performance is MUCH worse in hot weather because of the decreased density of the fuel/air mixture. You are stating essentially the opposite. Or at least adding a bunch of relatively minor variables as the cause when the real factor is fuel/air density. I’d recheck your math…

    • wxbrad

      Chuck, fuel density is higher in cold weather you are correct but that actually would mean you are using more fuel when its’ cold. My math is correct because the combustion isn’t what’s affecting your fuel economy when it cold. It’s the engine temperature, tire pressure, aerodynamic drag, power usage, fluid viscosity etc.. None of these are impacted by the air/fuel density. They all simply need more heat to work than in cold weather. I also think comparing a plane engine to a car engine is like apple to oranges Altitude is playing a much larger role in aircraft. In warm weather lift is actually much harder to achieve due to the lower density of the air. So there are other issues that impact planes than cars.

      • http://twitter.com/cshotton Chuck Shotton

        Actually, it means that the energy density of a gallon of gas is much higher in Winter than in Summer. It means that your engine will develop the same power output at a lower throttle setting, using less fuel to do so. So actually, it means that you burn fewer gallons of gas to create the same amount of power in Winter. Still think you need to check the math…

        • wxbrad

          Again you are refereeing to something that has no bearing on warming an engine What does the cooling system have to due with warming? It’s takes gas/energy to warm all components of the engine. If you are starting at a lower temperature you must then use more energy to reach optimal engine temps of 150°-195°. That’s before you even factor in drag, viscosity, idling and electrical usage. Trucks do use obstructions as well as NASCAR to regulate the temperatures. You can even well use of an engine block heater to help combat this. Though I don’t know a single person who has one, so those people are in the minority.

          • http://twitter.com/cshotton Chuck Shotton

            Normal auto engines would overheat without a cooling system. They obviously generate more waste heat than is necessary to maintain an optimal CHT, hence the need for water pumps, radiators, oil coolers, etc. So that means when it is cooler outside, the engine has to expend less energy running a water pump, the electric fan, etc. to maintain ideal CHT. So yet again, it’s more efficient when it is cooler.
            (I’m gonna forego any further discussion since the points I am making are contrary to the thesis of the article and therefore rejected, despite their accuracy. Specious issues like air friction/density or oil viscosity are clouding the one overarching consideration, which is efficiency of the fuel source. Denser fuel and higher oxygen concentration per cubic volume offsets all of these. In any case, engine oil is at a constant temperature after the first 5 minutes of engine operation. Parasitic drag on a modern auto body is more or less a constant as far as air density is concerned, at least within the altitude range that cars operate. Certainly less of a factor than the increased power output from a denser fuel/air mixture due to cold weather.)

          • wxbrad

            Chuck you are dismissing the fuel needed to achieve ideal operating temperature when the engine is below 68°. That combined with all the other factors listed averages to a 22% loss in fuel inefficiency. If you have research or studies that refutes that automobile get lower fuel economy at lower temperatures I’m willing to listen. Most of my information was pulled from engine manufactures and automobile companies and oil companies websites. Remember “winter” gas isn’t the same blend as “summer” gas.

          • wxbrad

            Actually raise your concern about the cooling system and water pump at colder temperatures. He actually told me that leads to more fuel being used.

            Synthetic oils are still pretty thick when its really really cold out.
            The main issue is still the radiator bleeding off the heat the engine needs, and the thermostat staying mostly closed because of the coolant cooling off so quickly.

            “When the thermostat is allowed to stay open all the time, the water pump doesn’t have to work nearly as hard, because of the resistance in the coolant flow that the thermostat creates when it is closed.

            That drag on the coolant pump requires the engine to work harder to keep spinning the serpentine belt on the front of the motor, which contributes to the loss of fuel economy that you are seeing.

            So not only is there a loss with the extra friction created by the colder air that you have to push your car through at highway speeds; and the extra friction created by the cold, thick fluids that your engine and transmission have to spin through; but the water pump has to work harder because the thermostat is staying closed most of the time.

            All of these factors add up to less fuel economy”

          • nope

            Until a modern engine comes to temp it runs rich to heat up the emissions system, hence the lower temp it starts the more fuel it uses. Very few engines use an electric or clutched water pump, so parasitic losses from the cooling system are fixed. I can see that you would “forego any further disucssion” since you have no idea what you’re talking about and have embarrassed yourself by posting this nonsense. Air resistance makes an enormous distance at speed, and denser air will cause an increased load. Colder oil flows more slowly. These issues are “specious” to you? You have no idea what you are talking about – which is obvious as you continue your post.

            “Denser” fuel? Really? Because of the extraordinarily small amount of contraction in fuel volume across winter temperatures. Why don’t you go ahead and give us the mass difference between a volume of fuel at 100F and at 0F. While you’re at it give us the comparison in BTUs between winter and summer blend gas for the Northeast. Here is a hint, lower BTU content HUGELY outpaces any increased density – that means LOWER fuel economy, not some fuel economy increasing feature that will cancel out other sources of lost efficiency. BUT, my favorite part, is that you keep going with the demonstration of your ignorance. “Engine oil is at a constant temperature after the first 5 minutes of engine operation”? HA! In what vehicle? You have obviously never monitored your engine oil temp. I doubt you can find a single vehicle that would have engine oil to temp within 5 minutes in winter, and *many* if not most vehicles will never have engine oil to temp in 5 minutes in any temp. Colder temps unequivocally will cause increased drag, which will increase with the square of the speed. So these will OBVIOUSLY make a noticeable problem in the winter. Did you even bother thinking before you posted? Your conclusion would certainly indicate the answer to that question is a resounding “no”. Please tell us, HOW ON EARTH COULD INCREASED POWER OUTPUT *EVER* LEAD TO INCREASED ECONOMY WHEN OVERCOMING INCREASED LOAD. Do you have any understanding of bsfc? Increased power = increased fuel usage.

            I strongly suggest you spend some time and go to school, and perhaps get a clue. Otherwise never post again. You have absolutely no clue what you’re talking about.

  • http://www.facebook.com/profile.php?id=1414547845 Brad Hancock

    The calculation on #2 is incorrect. Assuming .4%/PSI is accurate, 3 PSI loss would be 1.2% lower MPG, not 1.2 MPG, which is what you calulated. The MPG loss would only be 0.26 MPG.

    • wxbrad

      Good catch Brad you are correct fixing it right now.

    • Randy

      Thanks for pointing that out. That was the number I was curious about. I was curious how tire pressure affects efficiency. I have the tire pressure lower in my car to add traction. But inflating them some could bump up range…

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