Lower Winter MPG

[96Firebird]

So I keep extensive track of my mileage, and looking at the trends, I always get lower MPG in the winter. I know there are a multitude of reasons for this, including winter gas, winter tires, dense cold air, snow-packed roads, etc... So, which of these factors do you think contribute the most to lower MPG in the winter?

Looking at just this winter season, I think winter gas has the largest impact on mileage. I tend to put my winter tires on very late in the season, mainly due to laziness. Before even putting my winter tires on, my fuel mileage has been dropping. I don't think it has anything to do with snow either, since we have barely gotten any this year. Another common reason for lower mileage in winter is that the car takes longer to get to optimal temperature, but I drive almost an hour to work and back each day (~45 miles one way), so I don't think that contributes much to my situation. Does winter gas really drop mileage as much as I think it does, or am I going crazy?

Here is my fuel mileage plot, blue is when the winter tires go on, red is when the summer tires go on:

And here is the plot from my old daily driver, don't have data for when tires were changed:

 

[Raizinman]

Could be winter weight. People tend to gain weight in the winter.

 

[Jumpem]

It is the winter gas.

 

[nOOky]

Winter fuel blend. I've seen my mileage drop when switching to it and the actual air temperature and everything stayed the same so it was the only change. I'd guestimate about 5-8% lower mileage using it.

 

[SparkyJJO]

x3 on the winter gas. Back when I used to drive 30k+ a year and filling the tank every other day I could identify a bit better when winter gas hit the stations because even though temperatures probably either haven't really changed much yet (or in some cases already did change), I would notice a quick drop in mpg in just a tank or two. Same would happen in the spring when they switched back, mpg would come back up. Sometimes I got a little bit of yo-yo effect caused by some stations getting their winter blend a little later than others and I didn't fill at the same stations consistently (side note, my vehicle didn't really seem to care much at all what brand gas I put in it unlike the prior one that was noticeably better with BP gas for whatever reason).

 

[Yuriman]

On my particular car, my winter tires have only slightly worse rolling resistance than my summer ones. I might lose 5-10% there.

Winter slush probably causes the largest drop, but it usually clears up within a few days of a big snowfall.

My biggest factor overall is actually temperature. It's about 15 miles to drop my wife off at work, and on the days I don't go in as well, I might end up making the trip to and from town again immediately. When it's 10 below zero, I might get 55mpg there, 65mpg on the way back, 75mpg on the way there again, and 80mpg on the second trip back. Even though the coolant reaches maximum temperature within ~5 miles, it seems the other fluids and components are still getting up to temperature even after 60 miles.

 

[Eug]

Temperature of course is the primary issue. This is my 2004 Prius from 2004 until 2015, when it got totalled:

Units are in L/100 km, so the number is way higher in the winter than the summer, because I use more gas to travel 100 km in the winter. Note that my overall average of about 6.x L/100 km is lower than many in the US report, for a number of reasons (colder weather, shorter commutes and in stop-and-go traffic, driving style, etc.). In the summer I was more like 5.5 L/100 km, and in the winter I was more like 7 or even 8 L/100 km. That represents a whopping 25-45% difference in gas mileage.

Also, for my 2012 Plug-in Prius (purchased in 2015), when it's 0C out, I can get to work mostly on electricity if I don't turn on the cabin heater. However, if I turn on the cabin heater, the gas engine kicks on to warm up everything. This is pretty much a necessity if it's -10C out.

In addition, in EVs where there is no gas involved at all, the range of the battery in the depths of winter is roughly anywhere from 50% to 75% of the summer range. BTW, as an aside, my cell phone shuts off completely after one minute in the cold now. That's an extreme example, but a lot of technology, including cars, just don't like the cold so much.

 

[RU482]

The OP's top plot for the Buick curiously reflects the warmer winter we've had this year, and to a lesser degree last year

 

[96Firebird]

The OP's top plot for the Buick curiously reflects the warmer winter we've had this year, and to a lesser degree last year

Most of that is due to a change in driving. Around January last year I was laid off my job and didn't drive nearly as much, this winter I am driving 90 miles a day, pretty much all highway, which is why my average MPG is higher. Also, I fill up less during the summer because I have a summer car, and only drive the Buick on rainy days.

 

[repoman0]

How long are your trips on average? I think it has more to do with extra fuel usage on very cold starts. The car will run rich for the first couple minutes if the engine temp is below a certain threshold, and the huge decrease in fuel economy for the first 10-15 minutes of your drive probably contributes more to your reduced mileage than other factors.

My fuel economy drops in the winter as well, but my average trip (to work and home) is only about 25 minutes. On long drives, highway mileage is the same as in the summer, once everything is heated up and working at maximum efficiency in closed loop.

 

[Eug]

My best mileage is when I have no heater on and no air conditioning.

 

[Yuriman]

How long are your trips on average? I think it has more to do with extra fuel usage on very cold starts. The car will run rich for the first couple minutes if the engine temp is below a certain threshold, and the huge decrease in fuel economy for the first 10-15 minutes of your drive probably contributes more to your reduced mileage than other factors.

My fuel economy drops in the winter as well, but my average trip (to work and home) is only about 25 minutes. On long drives, highway mileage is the same as in the summer, once everything is heated up and working at maximum efficiency in closed loop.

It's more than just the engine running rich, though you're right that it will run rich for longer, the longer it takes to warm up. Rolling resistance is higher, fluids are thicker, air is more dense (which causes higher fuel consumption for several reasons), and your pistons and engine block may not have finished expanding, resulting in more mechanical friction.

Tires and fluids (engine oil, grease in axles, transmission fluid) may not all be up to temperature by the time your coolant is, but most vehicles don't have sensors for these things. Cooler fluids are generally more viscous and will rob economy to some degree or other.

~

When I'm running my heater on max without recirculate (to keep condensation down), it's actually not uncommon for engine coolant temps to start dropping, even when I'm on the highway, when it's -10 or -20F outside.

 

[Herr Kutz]

That's good mileage for an 03 Park Avenue. You must be using premium gas.

 

[ipown1337]

Overall I believe that since cold air is more dense the engine naturally makes more power and a richer fuel mixture is required.

 

[Bubbleawsome]

Plot the graph against tempurature and you'll more easily be able to see if it's mostly coledfluids or winter gas, or both.

 

[disappoint]

Denser air requires more fuel to reach stoichiometric combustion.

 

[Zorba]

Denser air requires more fuel to reach stoichiometric combustion.

But doesn't a modern fuel injected car account for that and suck in less air? Denser air = more air (for same volume) = more fuel = more power generated = has to go somewhere (either motion or heat)

Really even a carburetor would in that you need less throttle to get the same output power.

Aerodynamic drag is proportional to air density, though, so the denser the air, the more drag. This is the reason airplanes expend a lot of fuel to get to high altitudes where the air is very thin.

 

[tortillasoup]

Lower Intake air temperature = more dense air = more fuel burned in order to maintain proper air - fuel ratio. Warm up times also have a large effect on fuel economy. My fuel economy on my cars when it's 60F outside, cruising on the freeway has been about the same as during summer time even with using winter fuel. Check tire pressure to make sure it hasn't dropped to lower than usual. Tires lose pressure as the temperatures drop so adding air is absolutely necessary when it gets cold outside (if it has until recently been moderate temperatures).


One thing hypermilers do is route the intake air to near the exhaust manifold to increase the intake air temperature, improving fuel economy due to less dense, air being burned.

But doesn't a modern fuel injected car account for that and suck in less air? Denser air = more air (for same volume) = more fuel = more power generated = has to go somewhere (either motion or heat)

The only way to account for the additional power produced by more dense air charge is to restrict the throttle further which adds to pumping losses. This is why cars get better fuel economy at high altitudes vs. lower altitudes even not considering air resistance when driving. The idle fuel consumption of these vehicles is lower at high altitudes vs. sea level.

 

[Yuriman]

The only way to account for the additional power produced by more dense air charge is to restrict the throttle further which adds to pumping losses. This is why cars get better fuel economy at high altitudes vs. lower altitudes even not considering air resistance when driving. The idle fuel consumption of these vehicles is lower at high altitudes vs. sea level.

This. Throttle plates generate vacuum, which isn't cheap, and a car generating more power at the same throttle opening will have to close the throttle more. Honda did away with throttle plates in some of their Civic models in the early 2000's in order to minimize this parasitic loss, but I'm not sure if they're still doing it.

 

[RLGL]

There was a spot in my area that sold fuel that I regularly got more mileage year round than the competitions fuel. This year when the winter blend hit the pumps, my mileage dropped like a rock. I found out they changed suppliers. Now my mileage is the same no matter where I purchase the fuel. Moral: It is not only the seasonal blend but also the quality of the product.
And yes the outside temp does affect the mileage, the mapping of the computer is such that when it is cold or extremely hot the mileage will change.

 

[tortillasoup]

This. Throttle plates generate vacuum, which isn't cheap, and a car generating more power at the same throttle opening will have to close the throttle more. Honda did away with throttle plates in some of their Civic models in the early 2000's in order to minimize this parasitic loss, but I'm not sure if they're still doing it.

wut? never heard of this for Hondas.

 

[Yuriman]

To understand the design objective of the R18A SOHC i-VTEC, we look at light throttle situations described above, when the car is running leisurely or at 'light load' (no hard demands on power or performance). In such conditions, the engine often spins lazily at around 2500 to 3500rpm and throttle pedal pressure is extremely light. Consequently the throttle butterfly is almost closed, especially at legal highway speeds (between 80, 90 or at most 110kph in most countries). Now when the pistons are running, they will need to suck air from the intake manifold and through the almost closed throttle butterfly. However the almost closed throttle buttefly will be resisting the sucking / pumping action of pistons. This wastes energy and is called "Pumping Loss".

If we think about it, this situation is actually very ironic when we look at it from the practical point of view. Here is the situation where we are able to, and expects the engine to produce great fuel economy. But the almost closed throttle butterfly also introduces the largest amount of pumping restriction - and power loss - to the engine. So the almost closed throttle butterfly actually negatively impacts the engine's operating efficiency and thus its fuel economy, at a time where the operating environment is most conducive for the engine to deliver good fuel economy.

In a case of clearly thinking outside of the box, Honda's R&D engineers for the Rseries tackled this imbalance directly and introduced the Rseries's SOHC i-VTEC implementation. So the Rseries SOHC i-VTEC is a fuel economy implementation that works by targetting at the reduction of pumping losses in the engine in driving conditions which are most conducive to fuel economy. Now the R18A engine can truly be seen as a 'dual mode' engine with clear-cut normal driving and 'economy driving' modes.

SOHC i-VTEC Implementation (R-Series Engine)

As explained above, under so-called low-load conditions (steady speed cruising within speed limits, down-hill, generally light to very light throttle driving) on a conventional engine, the throttle valve is normally partly closed to control or limit the intake volume of the fuel-air mixture. The air-flow restriction that is incurred due to intake resistance in such conditions is one of the biggest factor that leads to reduced engine efficiency, through pumping losses explained above. With the R18A SOHC i-VTEC mechanism, Honda's R&D engineers introduced the idea of controlling the amount of air entering the combustion chamber via other means instead of the throttle butterfly. This is done via a combination of both valve timing and an exploitation of the property of the 4-stroke cycle and a new Drive By Wire (DBW) system.

Source

Looks like it was first released in the 2006 Civic. Since it's not a performance feature, most people ignored it though.

EDIT: I realize what I said in my previous post wasn't strictly true, though - the throttle plate is still there, it's just not used sometimes.

 

[thomsbrain]

Winter gas cuts about 10% off your mileage. The rest is going to be tires, colder starts (with associated longer warm ups for oil, transmission fluid, axle/bearing grease, tire pressure, et cetera), rain/snow drag, and the increase in crappy traffic conditions. And don't forget most modern cars run the A/C anytime you have it in defrost mode, whether you ask for it or not.

 

[Mark R]

Cold air.

I did a bunch of calculations on drag factor, air pressure, viscosity and density - and calculated that this explained about 60-80% of the drop in mileage.

My local fuel supplier also posted analysis of the fuel on a monthly basis, and there was minimal difference in fuel density and aromatic content in Winter; not enough to make a meaningful difference.

 

[Subyman]

Well you answered your own question in the first paragraph. All those combine into noticeable change in mileage.