Fuel Economy Concerns during the winter
We seldom have multiple days of cold weather in the Puget Sound region; as such there have been a lot of fuel economy concerns expressed over the last few days, so I thought I would try to explain why your Subaru has such a dip in economy during the colder weather.
when the temperature is below 40 degrees it just takes the engine longer to reach operating temperature, and until it does you just can’t obtain optimum fuel economy, when the roads are slick the All Wheel Drive will engage much more frequently creating more load on the engine thus using more fuel. 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. 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 wind shield. If your Subaru has seat warmers you will probably turn them on, its darker longer this time of the year so the head lights are on longer than normal (if you ever turn them off).
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 inputted to the Vehicle Engine management Computer 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 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 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.
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 blender has 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”. 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.
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.
Things that are important all year long become can become crucial during the winter and summer months depending on your Climate.
Thanks for Reading
Winter formulation gasoline often has more Ethanol and/or other oxygenates which decrease the energy density of the fuel but can help decrease cold weather smog. The result can be slightly higher fuel consumption.
I like filling up at Spirit in North Seattle – one of the few local stations that still has 100% Ethanol-free gasoline – its a little more exensive than Shell/Chevron/etc.
Hello Karl,
Valid points, I wish we could get to 70mph During our Commute in the Seattle area.
The only thing I would really point out is once the car is driven the tires heat up very quickly even in the cold and the pressure in the tire raises as well despite the temperature outside.
It is definitely a contributing factor starting out just like in the case of the fluids. I guess some of what I was trying to point out were things we can do to combat the problems faced in colder weather as much as explain what happens.
Thanks for reading and contributing to the conversation
Justin
Two major factors that should be mentioned in your excellent article:
1) Aerodynamic drag on the car is greater in colder, denser air. I believe that in a long trip at highway speeds, this is a bigger determinant of low mileage than any of the factors mentioned. At 70mph, most of the engine’s power is expended imparting momentum to the surrounding air, which is significantly heavier at low temps.
2) Tire efficiency is much lower in the cold. Sidewalls and tread must flex as the car rolls, and this sucks much more energy when the rubber is cold, stiff, and stubborn. Depending on the tire size, rubber formulation, and pressure, this can be significant, and is compounded by the fact that when the temp drops drastically, so does your tire pressure, so the rubber flexes even more, and does so less efficiently.