Gas mileage dropping during winter months

[DSeid]

I always assumed that this part - “it takes longer for your engine to reach its most fuel-efficient temperature” - was key, especially given that many of us are commuting fairly short distances. But anyone know how long it takes a typical ICE to get up to optimal temperature in moderate versus cold weather?

In any case the op loses about 25% on his mpg only a bit more than the .gov cite claims is average, especially if there are short trips involved. CNBC quotes Oak Ridge Laboratories as finding a third off in some models.

[Kevbo]

@Joey_P

In theory you should get better mileage in winter. Denser air means more air in the combustion chamber and more expansion during the power stroke.

The density information is correct, but this should not lead to improved fuel economy, and in fact the opposite is true.

The efficiency and power at full throttle (WOT)ARE improved, but WOT operation is only a tiny fraction of a normal driving cycle.

When throttled back for normal driving, the increased air density requires the throttle to be closed farther, which increases pumping losses, and in a spark ignition engine these are quite significant, especially if the vehicle has a large (for weight) displacement engine.

The increased air density also increases aerodynamic losses in proportion to the density.

The cold also reduces tire pressure, which can adversely impact fuel economy if not maintained seasonally.

Next, the engine may be idled to warm the interior and defrost windows, where 0mpg is achieved. Even if not idled, the engine spends more time warming to operating temperature, and operating at richer-than-normal mixture, which reduces fuel economy. The cold intake charge may lead to poor vaporization even after the engine has warmed, which may impact fuel economy.

Cold lube throughout the drive-train also adds viscous drag.

Lastly, as mentioned, winter blend fuel has more light fractions, and may contain oxygenating additives (MTBE or ethanol) to reduce emissions which lower energy content, and reduce economy.

Motorcyclists with fuel injected bikes often notice improved fuel economy at high altitude. The relatively large displacement engines, poor aerodynamics, and high speeds tend to showcase the economic effect of reduced air density.

[pulykamell]

@Orwell

I mostly blame it on the winter fuel,

Before this thread and doing some preliminary research, I would have thought the same thing, but all the reliable sources I could find seem to say that the winter blend itself only accounts for about a 2% loss of mileage. So it most likely is the other factors. (I don’t warm my car up by idling, but road conditions and stop-and-go driving are just so much worse here in the winter, that I’m betting that’s where the majority of the 5 mpg or so difference comes in my case.)

[Machine_Elf]

@Rick

It doesn’t matter the density of the air. The engine control module is going to inject the correct amount of fuel to maintain the proper air fuel ratio. So more dense air = more fuel. On the other hand since for any given throttle setting more power will be generated you tend to use less throttle which tends to cancel out the more fuel thing.

This, in part. Less throttle (to compensate for increased ambient air density during winter) means a greater pressure drop across the throttle plate, increasing pumping losses. (Diesel engines have no throttle plate, so this isn’t an issue for them.)

Cold-start enrichment lasts somewhat longer during winter. For short trips, this will have a bigger effect on fuel economy.

Idling for extended periods to warm up the engine and cabin is not only bad for fuel economy, it’s bad for the engine. It will warm up faster if you put some load on it. Start the engine, let the idle stabilize for all of ten seconds while you fasten your seatbelt, and then drive away, using modest accelerator application and moderate RPMs until the engine gets somewhere close to normal operating temp.

Cold lubricants are more viscous, and therefore increase friction. This is true in the engine, transmission, differential drive, wheel bearings, and so on. Even the tires will exhibit greater viscous behavior when cold, increasing rolling resistance. Because of thermostatic control, your engine will eventually get up to same operating temp that it does during summertime, but this is not true of the rest of the driveline; basically, if it’s 60 degrees colder outside, then the peak operating temperature of those components will also be 60 degrees colder, and so they will exhibit more drag than they do in the summer.

Aerodynamic drag is greater in winter due to the increased air density. Comparing an ambient temp of 10F against an ambient temp of 70F, the winter aerodynamic drag will be 13% higher. This will be less of an issue at city speeds (here the constant accelerating/braking dominates) but on long highway cruises, your fuel economy will be affected accordingly.

[Really_Not_All_That_Bright]

In Florida, it works the other way around. We’re not using AC all the time so mileage increases slightly.

[Johnny_L.A]

@Rick

On the other hand since for any given throttle setting more power will be generated you tend to use less throttle which tends to cancel out the more fuel thing.

Good point. I forgot about that.

@Kevbo

The density information is correct, but this should not lead to improved fuel economy, and in fact the opposite is true.

…

The increased air density also increases aerodynamic losses in proportion to the density.

@Machine_Elf

This, in part. Less throttle (to compensate for increased ambient air density during winter) means a greater pressure drop across the throttle plate, increasing pumping losses.

Interesting points. My physics education is limited.

About the aerodynamic losses: I’ve noticed that headwinds have a measurable effect on my mileage. The Prius is a fairly ‘slick’ car, but it doesn’t have a very powerful engine; so headwinds (and crosswinds) do take more power. And it’s always a headwind or crosswind. Why do I never have tailwinds? :mad: (OK, I do; but more often I don’t.) Still, the Prius gets twice the MPG as the Jeep! Speaking of which, when I have the roof rack on, MPG is lower. I have a Yakima rack with a Load Ranger basket and a wind deflector. I can reduce drag by not using the towers and bars, and mounting the basket directly onto the factory rack.

[Machine_Elf]

@Johnny_L.A

About the aerodynamic losses: I’ve noticed that headwinds have a measurable effect on my mileage.

Aerodynamic drag force scales with the square of air speed. So if you’re traveling 75 MPH, a 5-MPH headwind means your drag will be 80[sup]2[/sup]/75[sup]2[/sup] = 1.14 times higher than if there were no head wind. However, as you put your foot down to open the throttle and provide more power, the pumping losses decrease, making your engine more efficient - so you won’t see the 1/1.14 fuel economy you might expect, the drop will be somewhat smaller. But yes, it doesn’t take much headwind to make a noticeable difference in fuel economy.

[Hari_Seldon]

The temperature ranges here in Montreal vary by about 40 deg. C (+30 typical summer high, -10 typical winter high), although the extremes are more like 70. So that means tire pressure will be at least 10% lower. But for me the biggest difference summer and winter is the number of short trips I take. From April through October, I will never take the car for trips under a mile unless I am carrying something (like groceries). In winter I take many more short trips. Since the car doesn’t get a chance to warm up, the mileage for those trips is terrible. I also drive up to the enclosed mall a mile from here just to have a warm, dry, non-slippery place to walk. Then a mile home. It all adds up to much poorer mileage.

[SpeedwayRyan]

I reset my trip meter (which has its own mileage calculator) on my WRX last time I filled up, and my fuel mileage, which is NEVER great on a turbocharged AWD car, has dropped dramatically due to severe cold here.

I have a very short commute – less than 2 miles – and I either warm up my car in the driveway beforehand, especially in the morning (getting 0 mpg) or drive while the car cold, especially after work, neither of which is helping much. The short commute, combined with all the factors listed on the link upthread, means that when it’s really cold out, my MPG seriously sucks.

If I take a longer drive, the average starts to increase pretty quickly…but I don’t do that often in my car.

[Rick]

@SpeedwayRyan

I reset my trip meter (which has its own mileage calculator) on my WRX last time I filled up, and my fuel mileage, which is NEVER great on a turbocharged AWD car, has dropped dramatically due to severe cold here.

I have a very short commute – less than 2 miles – and I either warm up my car in the driveway beforehand, especially in the morning (getting 0 mpg) or drive while the car cold, especially after work, neither of which is helping much. The short commute, combined with all the factors listed on the link upthread, means that when it’s really cold out, my MPG seriously sucks.

If I take a longer drive, the average starts to increase pretty quickly…but I don’t do that often in my car.

Wow with that kind of drive are you sure in the winter your trip computer isn’t reading Gallons Per Mile?

[vomit_comet]

I don’t know about anyone else but I always noticed that a carbureted V8 seemed like it was producing more power on a very cold day as opposed to a very hot day; this was back before winter blend fuel when you still had to add methanol yourself to your tank to keep your gas lines from freezing. If it had an automatic choke based on the temperature of the intake maifold, the cold are would keep the choke partially closed and cause the fuel/air ratio to come back up to match the denser air, I.E. use more fuel.

[vomit_comet]

@SpeedwayRyan

I reset my trip meter (which has its own mileage calculator) on my WRX last time I filled up, and my fuel mileage, which is NEVER great on a turbocharged AWD car, has dropped dramatically due to severe cold here.

I have a very short commute – less than 2 miles – and I either warm up my car in the driveway beforehand, especially in the morning (getting 0 mpg) or drive while the car cold, especially after work, neither of which is helping much. The short commute, combined with all the factors listed on the link upthread, means that when it’s really cold out, my MPG seriously sucks.

If I take a longer drive, the average starts to increase pretty quickly…but I don’t do that often in my car.

I would be concerned with condensation buildup in your oil system. A “short trip” only engine will build up moisture in it and without actually getting the vehicle out and getting the oil hot, there is no way for it to burn that moisture off. I hope you’re changing your oil twice a year, regardless of mileage.

Do you ever see a sort of coffee&cream colored sludge on your oil filler cap or dipstick?

[GreasyJack]

@SpeedwayRyan

I reset my trip meter (which has its own mileage calculator) on my WRX last time I filled up, and my fuel mileage, which is NEVER great on a turbocharged AWD car, has dropped dramatically due to severe cold here.

I have a very short commute – less than 2 miles – and I either warm up my car in the driveway beforehand, especially in the morning (getting 0 mpg) or drive while the car cold, especially after work, neither of which is helping much. The short commute, combined with all the factors listed on the link upthread, means that when it’s really cold out, my MPG seriously sucks.

If I take a longer drive, the average starts to increase pretty quickly…but I don’t do that often in my car.

Also, have you ever actually calculated it by hand when you fill up? Some cars don’t count idling time in their running MPG display, and so the actual mileage with your drive cycle might be even worse than you think!

[Rick]

@GreasyJack

Also, have you ever actually calculated it by hand when you fill up? Some cars don’t count idling time in their running MPG display, and so the actual mileage with your drive cycle might be even worse than you think!

What car doesn’t count idling in MPG calculations?
Cite please

[GreasyJack]

@Rick

What car doesn’t count idling in MPG calculations?
Cite please

What I’m basing that on is that on some cars the cumulative MPG doesn’t move when you’re idling. Obviously it’ll take a while if it’s averaging over a lot of miles, but right after you reset the counter (assuming you can) the MPGs should go down pretty fast when you’re idling. It does on some cars, but on others it doesn’t budge. It could be the computer on those cars somehow adds it back in later, but given how notoriously optimistic the dashboard MPG readings are I think it’s more likely they’re just not counting the fuel consumption at idle.

Obviously that’s some speculation, but it’s not like they publish the exact methodology they use to arrive at the MPG number as far as I know.

[DSeid]

The how …

… Roger Clark, senior manager of GM’s energy center, explains that the fuel economy gauge makes a calculation by counting the number and duration of pulses made by the fuel injectors as they squirt gasoline into the combustion chambers of the engine. The onboard computer system divides the distance the car travels by this estimated fuel consumption. …inaccuracies can come from such factors as fuels and even fuel tanks, which expand during warm weather. For instance, Kung notes that in the summer a Mini might be able to hold 14 gallons of fuel in a tank whose labeled capacity is 13.2 gallons. In the winter, it might only hold 12.9 gallons. This variation affects a variety of readings, including mpg …

[DSeid]

More …

… And then there are vapor-recovery systems. With modern cars’ sealed fuel tanks, gasoline vapor accumulated in the tank is absorbed by a charcoal-filled canister. Periodically, this canister is purged by the engine. “If the weather is hot, you generate lots of purge,” says Allen. “This unmetered fuel isn’t counted by the trip computer.” … Honda uses additional inputs to calculate trip mpg. “We look at the fuel-consumed data that comes from the engine-control computer, but we also track the float sensor measuring the fuel level in the tank,” says Raj Manakkal, chief engineer for electrical and infotainment devices. He also points out that, due to temperature changes, plastic fuel tanks can expand and contract by as much as a liter. On the Acura ILX, that yields a total variation of 4 percent. …

[california_jobcase]

I too have had cars with engines that made noticeably more power in cold, dry weather. The one that this was most noticeable in was my 1983 Datsun Nissan Sentra Diesel (yes, it had all those names strung in a line across its trunk lid). Of course, with 47 raging stock horsepower, any additional would be noticed!

Cheapest cost per mile car I ever owned.

[Rick]

@GreasyJack

What I’m basing that on is that on some cars the cumulative MPG doesn’t move when you’re idling. Obviously it’ll take a while if it’s averaging over a lot of miles, but right after you reset the counter (assuming you can) the MPGs should go down pretty fast when you’re idling. It does on some cars, but on others it doesn’t budge. It could be the computer on those cars somehow adds it back in later, but given how notoriously optimistic the dashboard MPG readings are I think it’s more likely they’re just not counting the fuel consumption at idle.

Obviously that’s some speculation, but it’s not like they publish the exact methodology they use to arrive at the MPG number as far as I know.

I have never seen a car where the trip computer didn’t count idling. This includes models from Mercedes, Volvos, Hyundai, Nissan, Ford and Mazda.
I’m sure you will agree this represents a fair percentage of the market.
That’s why I asked.

[Rick]

@DSeid

The how …

Either the author didn’t quote the guy from Mini correctly or he is an idiot.
To calculate MPG you need to know the volume of fuel burned and the distance traveled. Period. The gas tank has NOTHING to do with the MPG measurement.
Where the fuel tank does come into play is in calculating distance to empty. These reading can vary wildly. For a number of reasons.