Subaru Fuel Economy Concerns during the winter.

I started writing articles pertaining to Subaru repair for the website in 2007, some of the older articles get lost, I just dug this one out of the archives and added a few items to try and help explain the common questions we are asked right now about low fuel economy now that the weather has changed. Ive tried as usual to keep it Subaru specific and while some of the basic principles apply to all cars, the fact that Subaru’s are all AWD since the mid nineties (minus the BRZ) makes the things that affect fuel economy even more pronounced.

So here are some of the reasons why your Subaru has such a dip in economy during the colder weather.

When the temperature is below say 40 degrees it just takes the engine longer to reach operating temperature, and until it does you just can’t obtain optimum fuel economy. We tend to let the car idle longer in the am, or after the car has sat all day while we worked, in inclement weather we may have to let the car run for several minutes to help defrost the ice off of the windshield. Any time spent not moving while the engine is running negatively affects fuel economy.

Winter Fuel Blends.

Depending on where you live the fuel served up in the winter is going to be different than the fuel used the rest of the year. Winter gasoline often has more Ethanol and/or other oxygenates which decrease the energy density of the fuel but can help decrease cold weather emissions. The result can be slightly higher fuel consumption as the energy output of the fuel is decreased the amount of fuel to create the same energy is increased.

Increased electrical demand and engine load.

During the winter months the battery when cold takes longer to recharge after it is drained to start the car, the longer the alternator load to charge the battery the worse the economy will be during that time. Next when it’s cold you will use the rear defrost and the defrost control for the climate control more frequently the rear defrost is a large electrical draw creating alternator load, the front defrost causes the Ac compressor to cycle on to help hydrate the moisture off of the windshield, this is an extra load on the engine, if you have owned your Subaru for any length of time you know all too well know when climbing a mountain pass and the Ac compressor engage you notice it. The more you use the front defrost the more load on the engine the more load the worse the economy its a vicious cycle.  Its darker, longer this time of the year so the headlights are on longer than normal (if you ever turn them off at all) the longer the headlights are on the more electrical draw there is.

Tires and Alignment.

When the temperature outside drops, the temperature of the tires is also lower, increasing the resistance to flex in the tires, until the tires have warmed up, depending on the makeup of the tire this may be more pronounced on some cars much more so than others. Your tire pressure will always be lower when the tires are cold, which is why they are to be inflated to a max psi when cold, it would be an exaggeration to use the riding a bike with a flat tire analogy here and how much more effort it takes, but a tire that is even slightly low on pressure like it is in the cold will take more energy to roll just like you pedaling a low tire on a bicycle  If your alignment is off this will negatively affect the fuel economy in any temperature, but cold weather will compound this even further and add to the cold tire situation.  While the picture above shows an exaggeration of an out of alignment vehicle I think you can appreciate that a wheel that is pointing slightly to the left or right is resistant to going straight.  Lastly when the roads are slick the All Wheel Drive will engage much more frequently creating more load on the engine thus using more fuel.

Some technical stuff.

The colder air is denser and requires more fuel to maintain proper efficiency 14.7 to 1. When you add this to being at or near Sea level the effects are huge. Your Subaru has an air temperature sensor and the data from the sensor is inputted to the Engine Control Module  (ECM), which uses this data to determine the proper on time or pulse width of the fuel injectors, this is also done with other data inputs as well such as throttle percent, coolant temperature and load. The O2 sensors don’t start creating data for the Computer until they reach the proper temperature, most 1996 and newer vehicles have O2 sensor heaters, but just like your house, the colder the starting temperature the longer the on time of the heaters to reach the preset temperature, which also creates a huge amperage draw on the electrical system especially on the 2005 and newer cars as there are 4 O2 sensors to heat up.

As I mentioned earlier the colder the air the more dense it is, and the Aerodynamic drag on the car is greater in colder, denser air. So if you are lucky enough in your commute to travel at freeway speeds the colder air does affect the aerodynamics of the vehicle when you are traveling at freeway speeds this can be a factor in lower fuel economy.

Fluids are a Drag.

You have probably seen terms like 5w30, or 10w40 on oil bottles, what is commonly mistaken is that the “W” pertains to the weight of the oil. It stands for winter; during cold weather we need the oil to do its part to reduce friction. Think of it like this you put a few scoops of your favorite of ice cream in a blender and the blades of the blender have a hard time trying to turn in the thick substance, now add some milk and the blender picks up speed. Straight 30 oil is like ice cream in the blender, the mechanical rotating assembly really has a hard time turning. Add a dual property to that oil and you have oil that is safe to use in winter. While the engine is running the oil slowly starts to reach temperature, but not the transmission or differentials, it’s not until the car is driven for a bit does the gear oil and transmission fluid start to “thin out a bit” resulting in less engine horsepower and torque to rotate all of the moving parts. The dual property of a 75/90 gear oil helps protect the gears and bearings but it does still increase drag a little bit which creates a situation where it takes just a little more throttle to move the car than it would if it was at 50 degrees, this increases fuel consumption as well. Put one bottle of syrup in the Fridge, and the other in the pantry, now take them both out and have a race while you pour them out over waffles..

This is one of the reasons I always suggest starting winter out with fresh fluids as fluids break down or collect moisture the ability to lubricate diminishes. . Slow moving oil through a restricted oil filter can create unwanted engine wear as the oil struggles to reach all of the points in the engine before the RPMS are too high. Too much is made out of this or that oil being better, or “I can drive 10k on my synthetic oil and it makes my engine invincible”. If you are due for an oil change the oil, if you haven’t changed the transmission of differential fluids in a while change them. Want to know why some transmissions go the life of the car and never need repairs and some barely make it 4 years, it’s because one or more components was starved for lubrication as a result of a fluid issue.

Aged Components.

Back to the problems that arise with cold more dense air, is it makes the job of creating spark in a cylinder to ignote the air fuel mixture a little more difficult as well, as such if its been a few years since you have replaced your spark plugs, the cold weather will expose the deficiencies much more so than would have been present at the end of summer.  02 sensors that are slow to heat up because the heater element in the sensor is aged, or if the sensor it self has become”lazy” will have an affect on the economy in the winter.

The point of the article is to let you know that.

• 1. Fuel economy is going to be worse in the winter and why.
• 2. There are steps you can take to lesson the drop in economy, they will have a cost but have the added benefit of being good for your Subaru.

Thanks for reading.

Justin

Helping you get more from your Subaru.