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Old 12-10-2012, 12:13 PM   #1
Innovative Tuning
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Default Why your fuel economy is so much worse in the winter.

Each winter we at Innovative Tuning see posts on the internet and field concerns from our customers who often think they may have a significant issue with their vehicle based on a change in fuel economy. In the vast majority of cases there is nothing wrong with the vehicle. Instead there are a number of factors combining to reduce fuel economy. Rather than just saying it's cold out and that's why fuel mileage is poor, I'm going to explain how colder temperatures result in reduced efficiency in hopes that it will put people at ease and help them understand that much of the reduction in efficiency is unavoidable.

First I'll cover things that are inevitable:
1) Increased aerodynamic drag

Car companies invest millions to tune aerodynamics for a combination of stability at speed and low drag. Despite their best efforts they can't avoid the fact that air density increases as it gets colder. More dense air causes more drag. We can't see this so I'll give you an example that will hopefully help you visualize this.
Think of how conventional engine oil is thick when cold and visibly much thinner when hot (more on this later). Then picture wading through a pool of cold oil vs. hot oil. Don't picture the skin cancer or burns this may cause…just how much more energy it would take to walk through the cold thick oil vs. the hot thin oil.
In vehicle terms, as the car cuts through the more dense air, you require greater engine output to achieve the same speed, and higher engine output requires higher fuel and air consumption.
Air density increases ~20% when the temperature goes from 90 F to 0 F (other things being equal).
Aero drag is directly proportional to the density of the fluid or gas an object is passing through. This means a ~20% increase in air density results in a ~20% increase in aero drag.
This increase in aero drag reduces fuel economy more at highway speeds where aero drag is more significant than during around town lower speed driving.

2) Increased idle time
When your car is idling you're using fuel without going anywhere so you're getting 0 MPG. In the winter as we let our engines warm up before loading them during driving to reduce engine wear. This increase in time at idle increases fuel used before we start driving and drags down your average MPG.

3) Lower average engine temperature

When it's cold out it takes longer for an engine to warm up.
Engines are built with clearances that are meant to be optimal when the engine is warm. When the engine is cold, clearances are larger because components have not yet expanded to their normal operating clearances. This extra clearance results in greater emissions and reduced efficiency.
MPG is further decreased due to lower average engine temperature because engines are tuned to operate at richer air/fuel ratios during warmup. This aids in idle stability and smooths engine operation if an operator begins driving while the engine is still cold.

4) Engine and drivetrain lubricants are thicker when cold
As mentioned above if you've ever drained hot oil from a car vs. poured it cold you know it's thicker and resists motion more when it's cold. If you haven't done that, perhaps you've put cooking oil in a skillet and moved the skillet around to spread it out, then noticed how the oil moves around much more quickly/freely when hot.
Your engine, transmission, differentials (and other drivetrain components i.e. transfer case if applicable) are oil lubricated.
Until these oils come up to operating temperature they're thick and create significantly more drag on moving parts. Again this increased drag means more engine output is required to move the car forward and increased engine output means increased fuel consumption.
In addition to the oil lubricated components, your wheel bearings and other grease lubricated rotating parts move less freely while the grease is cold and more dense. Think of heating up hot fudge so it runs like water and then watching it get thick when it gets cooled by your ice cream.

5) Increased rolling resistance due to temperature change

In addition to the engine and drivetrain experiencing greater drag from thick fluids, your tire to road interface is experiencing increased rolling resistance in the cold. This change does not have a massive effect on fuel economy but it's worth mentioning.

6) Increased electrical loads
I'll keep this section simple as well since this doesn't have a huge affect on fuel consumption. Cold weather starting drains the battery more so the alternator needs to charge it more. In the winter we run the electric defroster(s), we use our headlights more because there are fewer hours of daylight, and we use the heater a lot. If you use the a/c all the time in the summer that balances with the heater usage. If you have heated seats, mirrors, front windshield, headlights, etc. those things all add to the electrical load. As electrical demands increase the alternator needs to charge the battery more and the alternator puts more drag on the engine while it does this. Again increased drag on the engine means more engine output is required so more fuel is consumed.

7) Winter gas

In the winter we are provided with altered gasoline formulas. They use a high vapor pressure formula which helps startup, but is less optimal in terms of drivability and efficiency when the fuel is warm. These fuels increase engine deposits and fuel dilution. Engine oil should be changed more frequently in the winter. If you'd like more info on this Chevron offers information to the public on their various gasoline formulas and other suppliers may do the same.

8) Road conditions
Driving through slush, snow, and ice increases rolling resistance so the engine has to provide greater output for the tires to push through it. These conditions often results increased traffic which results in reduced fuel economy as drivers speed up and slow down over and over. It's even worse in stop and go driving.



Here are some things you can do to help your fuel economy:
1) Tire pressures
If your vehicle has a recommended tire pressure of 35 psi and you drive with tires at 28 psi, you increase rolling resistance about $12% according to a study by TireRack. Tire pressure changes about 1 psi per 10 degrees Fahrenheit so it's easy for tires to be come under inflated as it gets cold. Some air leaks out of your tires over time as well so keep an eye on tire pressure because this is an easy way you can avoid a significant reduction in MPG.

2) Engine block heater / Heated garage
Engine block heaters warm the engine via an electrical heating element which you plug into a wall outlet. As it heats the engine block it also warms the coolant and oil. It reduces the density of oil and coolant which reduces drag on the engine. It improves the efficiency of combustion because the cylinders are warm to start. It reduces engine warmup time which reduces time spent idling at 0 MPG.
On a side note this also reduces engine wear which occurs on cold starts.
A heated garage won't get the engine block and coolant/oil as warm as a block heater, but it affects the whole vehicle so all fluids, bearing grease, tires, etc. start at a higher than ambient base temperature which will improve fuel economy too.

3) Narrower but taller snow tires to maintain tire diameter
Snow tires that are a bit narrower than your stock size will work better in snow and slush by increasing downward pressure and reducing the plowing effect as a wider tire pushes through snow. This improves traction and reduces contact area which reduces rolling resistance with the road and plowing resistance as the narrower tire goes through snow. Lower resistance means better fuel economy due to decreased engine output being required.

4) Wash car / Spray it off

Snow and ice collects on vehicles which increases vehicle weight, requiring greater engine output to move the vehicle. In some cases, snow and ice can build up so it rubs the tires which increases rolling resistance. Snow collecting on the under body and top side of the vehicle also disrupts the aerodynamics of the vehicle which increases aero drag. Spraying the snow and ice off reduces extra weight and returns the aerodynamic profile of the car to normal.

5) Engine braking
On vehicles equipped with manual transmissions, engine braking while engine RPM is above a certain point shuts the fuel injectors, halting fuel consumption. Engine braking may also provide increased vehicle control when compared to braking while in neutral. This works all year round.


We feel idle/warmup time, lower average engine temperature, winter gas and reduced tire pressure are the leading causes of reduced fuel economy out of all the causes listed. We feel maintaining proper tire pressure and using a block heater are the two ways you can best reduce winter MPG losses. Beyond that it's hard to prioritize the lesser factors because they're vechicle/situation specific.



Have fun, be safe, and happy holidays from all of us at Innovative Tuning!
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Old 12-10-2012, 12:23 PM   #2
I--RIDE--BIKES
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Wow that was really well done. Thanks!
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Old 12-10-2012, 12:29 PM   #3
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Winter tires also can also have a higher co-efficient of rolling friction. As soon as I switch to winter tires, I have a slight but noticeable decrease in fuel efficiency.
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Old 12-10-2012, 12:35 PM   #4
Innovative Tuning
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Quote:
Originally Posted by Grimoire View Post
Winter tires also can also have a higher co-efficient of rolling friction. As soon as I switch to winter tires, I have a slight but noticeable decrease in fuel efficiency.
They certainly can depending on which winter tires and summer tires you run, but that's not always the case so I left that off the list.
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Old 12-10-2012, 12:35 PM   #5
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Thank you so much for posting this... Best general post in a long while. This should be stickied!!!
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Old 12-10-2012, 12:38 PM   #6
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Excellent post. I'm always preaching to people as to why their MPGs can slide up to 15% in the winter. You posted some other legit factors I never considered.
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Old 12-10-2012, 01:33 PM   #7
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Great post, thanks.
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Old 12-10-2012, 02:09 PM   #8
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well done! I expected to find at least one item I could rip into... Nothing! Clear, concise, accurate!
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Old 12-10-2012, 03:24 PM   #9
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The only thing I was confused about is you said the tire to road contact will experience increased rolling resistance. I was under the impression that if you remembered to inflate your tires to proper pressure you would have lower rolling resistance. In the cold your tires will harden and have decreased traction so how is this more rolling resistance. This is assuming the only change is temperature and not having slush or snow or anything on the road.


I know you said it was practically negligible but still seems counter intuitive so further explanation would be appreciated.
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Old 12-10-2012, 03:24 PM   #10
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Good read, thanks.
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Old 12-10-2012, 03:28 PM   #11
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It's a nice informative post and probably a guideline in that anyone that didn't know any of that before probably shouldn't be working on their own cars.
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Old 12-10-2012, 03:52 PM   #12
Innovative Tuning
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Quote:
Originally Posted by ArkansasDave View Post
The only thing I was confused about is you said the tire to road contact will experience increased rolling resistance. I was under the impression that if you remembered to inflate your tires to proper pressure you would have lower rolling resistance. In the cold your tires will harden and have decreased traction so how is this more rolling resistance. This is assuming the only change is temperature and not having slush or snow or anything on the road.


I know you said it was practically negligible but still seems counter intuitive so further explanation would be appreciated.
Rolling resistance increases when a tire is cold vs. hot assuming equal pressures.
The reduction in grip can be drastically higher than the increase in rolling resistance.
When you break traction your MPG goes down further since engine output is wasted. (I'm not including the value of having fun playing in the snow in this discussion. :0)
I hope this clears things up. Good question.
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Old 12-10-2012, 04:08 PM   #13
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Great information from Buffaloians who know a thing or two about snow & cold weather
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Old 12-10-2012, 04:54 PM   #14
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Quote:
Originally Posted by Innovative Tuning View Post
Rolling resistance increases when a tire is cold vs. hot assuming equal pressures.
The reduction in grip can be drastically higher than the increase in rolling resistance.
When you break traction your MPG goes down further since engine output is wasted. (I'm not including the value of having fun playing in the snow in this discussion. :0)
I hope this clears things up. Good question.
Nope doesn't make sense. Read the whole article on Wikipedia on it. Says more tire wall deflection increases Rolling resistance but increasing temps decrease rolling resistance. Guess its just gonna be one of those things I don't understand but just is because it is. Oh well good post hopefully people will search and read before creating new threads.
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Old 12-10-2012, 07:41 PM   #15
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Informative. Makes me want to warm up before blasting out on cold days.
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Old 12-10-2012, 07:56 PM   #16
Innovative Tuning
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ArkansasDave on the plus side that's an extremely tiny portion of this equation so I wouldn't worry about it. Tire pressure is of vastly greater concern.
Quote:
Originally Posted by zoomnrx8 View Post
Great information from Buffaloians who know a thing or two about snow & cold weather
This made me smile. I have my studded snows on, but we're missing the snow thus far this winter.
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Old 12-10-2012, 09:05 PM   #17
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Thanks Innovative! My stromung sounds great! Thanks for the awesome open source!
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