Why does cold weather sap mpg?

[JEDI]

2015 mazda3i AT

10mile driving in city
70f: 35mpg
30f: 25mpg
10f: 15mpg

Ok, summer has summer blend gas.
But how about the difference between 30f & 10f? (Both in Dec)

 

[lxskllr]

Probably due to running richer, and you're barely getting to operating temp in 10 miles. There's also probably some small losses due to leaving the car running at stores and stuff while you get your crap together to leave the car.

 

[shortylickens]

This article explains a 12% drop, but yours is over 50%, which makes no darn sense.

http://www.fueleconomy.gov/feg/coldweather.shtml

 

[desy]

I see 5-10% usually

 

[HarryLui]

How long do you let it idle to get the warm heat?

 

[TennesseeTony]

Silly question......but do you keep the tires inflated properly in the colder weather?

 

[JEDI]

Probably due to running richer, and you're barely getting to operating temp in 10 miles. There's also probably some small losses due to leaving the car running at stores and stuff while you get your crap together to leave the car.

ahh.. operating temp.
thx

How long do you let it idle to get the warm heat?

none.
ive read no need in modern cars?

Silly question......but do you keep the tires inflated properly in the colder weather?

how much lower is the psi from 30f to 10f?

 

[HarryLui]

Someone is stealing your gas.

 

[ponyo]

Someone is stealing your gas.

About the only logical explanation.

 

[lakedude]

I think a big issue is the viscosity of the lubricants, especially the spots that don't get warmed from engine heat like wheel bearings. My garage is slightly sloped. In the Summer the car will roll out of the garage in neutral with no power. In the Winter the car is going nowhere without gas. My Civic had a manual transmission and I coasted much of the time. It would roll twice as far in the Summer as the Winter. It was like the brakes were on in the Winter.

 

[Meghan54]

I think a big issue is the viscosity of the lubricants, especially the spots that don't get warmed from engine heat like wheel bearings. My garage is slightly sloped. In the Summer the car will roll out of the garage in neutral with no power. In the Winter the car is going nowhere without gas. My Civic had a manual transmission and I coasted much of the time. It would roll twice as far in the Summer as the Winter. It was like the brakes were on in the Winter.

While true lubes tend to "thicken" when cold, I'd be willing to wager the lubes in a vehicle not subject to engine heat, like wheel bearings, U-joints, etc., are at normal operating temps. within a mile.

 

[Yuriman]

Lots of correct answers here. In the summer I typically get between 80 and 100mpg on the highway. I'm struggling to get 65 with the 0 to -10F weather we've been having, and without any modifications to my car, I'd be seeing upper 40's to lower 50's.

Off the top of my head, some factors:

Combustion is less efficient at lower temperatures. You can improve this by building a warm air intake. I have a piece of dryer duct which I run from my air filter box to near the catalytic converter during the winter. When the car is fully warmed up, I can see intake air temperatures in the 80's to 90's with an outside air temperature of -10F.

Cold air is more dense, and more dense air = more power (e.g. cold air intake) = the throttle plate is less open to generate the same amount of power = more vacuum = wasted energy. A warm air intake can help this too.

When the air is cold, you have marginally higher aerodynamic drag.

Cold engines are less efficient for a variety of reasons, and when it's cold out, they start colder and take longer to warm up. They're designed to have certain tolerances at operating temperature, and those tolerances are different before the metals have expanded, so friction is higher. Cold cylinder walls absorb more heat from combustion, meaning you get less of it turning the crankshaft and more of your combustion energy dumped into your coolant. The warmer your cylinder walls, the less energy is lost this way.

I have a block heater and an oil pan heater in my car, but I can't use them this year due to not having an outlet available. They help significantly in shorter trips. I also have a grille block to direct less of the frigid air through the engine bay. You can cut up a piece of cardboard or use a trash bag, and partially (or fully) cover your grille, but I suggest you keep an eye on temps for a while if you cover more than, say, half of it. I can personally 100% block my grille when it's below freezing and even then, the thermostat doesn't always fully open.

When cold, lubricants are thicker. When it's -10 outside it feels like I'm pushing jello through my clutch line, and my shifter feels stiff for at least the first 10 miles. You can bet the same is true of antifreeze and engine oil, and as parts move inside the engine, they have to push this thicker fluid out of the way. I run what's effectively at -10w15 in my engine here and it helps. (FWIW The car has had this oil for nearly the last 250,000 miles, it still has perfect compression and Blackstone hasn't found anything unusual in my oil samples so it's not causing excess wear. It shouldn't though, considering it's what Honda calls for in most of their vehicles in Europe and Japan.)

Cold tires have higher rolling resistance. Slush on the roads increases rolling resistance, much like standing water.

As others have suggested, cold air causes your tire pressure to drop, and you're also likely spending more time idling.

While true lubes tend to "thicken" when cold, I'd be willing to wager the lubes in a vehicle not subject to engine heat, like wheel bearings, U-joints, etc., are at normal operating temps. within a mile.

My testing suggests otherwise. On a flat road, I've found my fuel economy continues to rise for at least 40 miles, despite coolant temperature peaking after ~5-10.

 

[RLGL]

how much lower is the psi from 30f to 10f?

Typically tires lose one pound of air pressure a month.
Form my summer highway high of 42 mpg, I go to a low of 28 city in the winter.

 

[bigi]

Compensate 1 psi per each 10 F degree. If you are supposed to be at 35 PSI at 70 degrees, then do 42 when you expect to drive at 0.

 

[RLGL]

Compensate 1 psi per each 10 F degree. If you are supposed to be at 35 PSI at 70 degrees, then do 42 when you expect to drive at 0.

Do not over inflate the tires to compensate for temperatures. That can affect tire life and handling characteristics.

 

[who?]

Regularly check the tire pressure whenever the temperature changes a lot and always try to get it to the pressure recommended on the sticker on the drivers door or doorframe. Don't try to compensate for temperature.

 

[lakedude]

Wow Yuri-dude, you really know your stuff.

Regarding tire pressure, I suspect that under-inflating is no better than over-inflating. Because PV=nRT pressure will be lower in cold temps period. If you air up in the cold than your pressure will be a little high when the temps are high and vice-versa. The only way you will always have the perfect pressure is if (unless you have central inflation) you check and fill several times a day to compensate for the outside temps. Since this is not really practical your pressure is always going to be a little off one way or another as the temperature changes.

Megan54 I really don't see how for example wheel bearing grease is going to warm up in a minute in say freezing temps. Friction is the only source of heat. Meanwhile the whole wheel bearing is surrounded by a huge rotating heat sink (the wheel and hub). I expect wheel bearing grease stays quite cool for quite a long time in freezing temps. In fact I bet it never warms up to anywhere near Summer temps in the cold of Winter.

The engine is another matter. Engines produce enough waste heat that they will require cooling even in the Winter.

 

[zerocool84]

Are we also forgetting about summer and winter blends of gasoline?

 

[JEDI]

Are we also forgetting about summer and winter blends of gasoline?

um.. didn't I mention that in the OP?

 

[repoman0]

My testing suggests otherwise. On a flat road, I've found my fuel economy continues to rise for at least 40 miles, despite coolant temperature peaking after ~5-10.

That could partly be due to your engine oil continuing to warm up as well. My car has both an oil temp and coolant temp gauge and the coolant warms up significantly faster than the oil -- about 5-10 minutes, as in your experience, vs. ~30 minutes. Of course it also takes 6 quarts and I'm guessing the insight takes a lot less.

 

[Yuriman]

That could partly be due to your engine oil continuing to warm up as well. My car has both an oil temp and coolant temp gauge and the coolant warms up significantly faster than the oil -- about 5-10 minutes, as in your experience, vs. ~30 minutes. Of course it also takes 6 quarts and I'm guessing the insight takes a lot less.

It takes ~2.6 quarts and has a coolant to oil heat exchanger. I don't have a way to monitor oil temps though.

 

[Scarpozzi]

I'll throw out another one....the air going through the intake isn't typically just colder, but it's typically drier too. I'm not sure if that makes a difference, but it could affect the MAF and the O2 sensors if they were calibrated for 50% humidity. If it affects torque, it takes more rpms to get the same power.

 

[Yuriman]

I'll throw out another one....the air going through the intake isn't typically just colder, but it's typically drier too. I'm not sure if that makes a difference, but it could affect the MAF and the O2 sensors if they were calibrated for 50% humidity. If it affects torque, it takes more rpms to get the same power.

The torque/RPM bit is true in some cases, but not others. On a CVT where an engine is basically always running WOT (assuming the CVT has a wide enough range) and just varying RPM to get the correct amount of power, losing torque will increase RPM, which usually means lower economy. However, on manuals and all other automatics, anything that reduces torque will typically increase economy. The reason for this is, most engines will not be WOT, meaning the throttle plate is part closed and creating vacuum. Lowering air density and causing the plate to open more to make the same amount of power will reduce vacuum, which is a parasitic loss. The fuel economy savings by reducing vacuum (all else being equal) are actually pretty significant.

 

[Scarpozzi]

The torque/RPM bit is true in some cases, but not others. On a CVT where an engine is basically always running WOT (assuming the CVT has a wide enough range) and just varying RPM to get the correct amount of power, losing torque will increase RPM, which usually means lower economy. However, on manuals and all other automatics, anything that reduces torque will typically increase economy. The reason for this is, most engines will not be WOT, meaning the throttle plate is part closed and creating vacuum. Lowering air density and causing the plate to open more to make the same amount of power will reduce vacuum, which is a parasitic loss. The fuel economy savings by reducing vacuum (all else being equal) are actually pretty significant.

When I hear CVT I just think about lawn mowers. I keep forgetting that so many japanese small cars have gone that route. Those didn't cross my mind because I've never owned one.

I was really just saying that lowered torque means the driver's going to hit the gas if they don't change their driving habits in cold conditions (meaning their desired acceleration and speed performance).

 

[Yuriman]

True, but hitting the gas doesn't necessarily burn more fuel. For instance, driving up a mountain, the air gets thinner. You press the gas farther as you ascend to burn the same amount of fuel because the air is getting thinner, and you need a fixed amount of air for cumbustion. Gas mileage would improve as you go up (assuming you don't have to downshift) because you'd have less and less parasitic vacuum loss as that throttle plate opens more.

EDIT: Am I going beyond the scope of the discussion?