whats the reason MPG is better in warm weather?

If you want good winter mpg you can get it, except for the winter fuel drop.
Guess you can eliminate that loss too if you save up some fuel to use from summertime!

But you have to:

1 Keep the engine warm with extra insulation and or radiator blocking as needed.
2 Take trips of like 3 hours or longer.
3 Go highway only, not slow roads that can't keep the engine warm. And minimize
stopping.
4 If you normally drive 70 mph go 50 mph, depending on temps to match air density.
5 Use a canview or equiv to monitor temps and make sure you use only waste heat for the cabin. This means stay cold till after the car is happily warm! And make sure you cut heat if car starts to cool. Also needed for item 1 so you don't overheat.
6 Only drive between storms when roads are clear and dry.


Do that you should match the summer mpg exactly minus the fuel energy differences. Around 8% as mentioned above.

But most of us take short trips, and slow trips, and the car never gets really warmed up. So there are lots of added losses.

Most have been mentioned here:

1 The fuel mix for winter fuel has less energy.
2 The mixture runs rich and feeds fuel for extra heat to warm the catalytic converters.
3 The air is denser so drag increases.
4 Lubricants are thicker and add to engine and drive train losses.
5 The car needs more heat to keep warm and loses it quickly on stops or engine off.
6 The battery is mostly shutdown till it warms up and this can take over 30 minutes. Even a block heater doesn't warm the battery. Cabin heat helps a small amount. How you set the heater controls has a big effect.
7 In city driving nothing will keep the engine and you warm no matter what so avoid city driving when really cold.
8 Around New England the roads are covered with sand, water, slush, snow, etc enough to add significant extra rolling losses.
9 It takes longer to get to stage IV and is harder to stay there. (really another way of stating items 2, 5, 6 and 7.)

My commute to work is about 10 miles.

Summer I get over 55 mpg
Winter I got typically around 40.
Winter with block heater I can get over 45.

But on trips in summer I have peaked around 59 mpg.
Winter trips I have regularly gotton over 55 mpg.

The only difference here is the trip lengths and I have more stop and go on my commute trips.

 

Evan this post by tomdeimos should be in the KB under fuel ecomomy as he has posted one of the best on winter fuel economy I've seen posted in a long time.
tomdeimos nicely stated.

 

Well put. Do you (or anyone else) have any idea what the relative importance of these 9 factors are? I feel like 1 and 2/5/7 (i.e. those that involve keeping the engine warm) are the most important, but I don't have any hard numbers on this. Cold air being denser will certainly have an effect, but will it be a signficant one?

 

Great thread -- great post.

Road condition is definitely a factor that I've observed. Get to observe again tomorrow -- unfortunately

 

Some things are hard to quantify, but if you have a regular route where most conditions are constant you can experiment and get a good idea of how big some effects are. I have done that with different heater settings and I quickly learned that for best mpg I needed to let the engine warm first, then some heat for me, when conditions like window fogging allowed. Then when there is enough heat I let the air flow through to the battery with recirc off and faster fan speeds, when I can keep things warrm.

Note that all effects are additive, so most all are important, but depending on conditions. For example, if the car stays just too cold for the engine to shut off at a red light, you are wasting fuel. But if you never have to stop at a light, this won't matter to you and your overall mpg.

Fuel for winter I think the 8% is about right, but this could vary with area and might be less or more where you are. Not much you can do here.

The temperature effects are big, and these affect you most for short trips. So again what is really important depends on your exact travel situation. You know this is big just from the low mpg for the first 5 min on the Toyota graph. For me it is often under half what I get after warm up. The overall effect from this depends on the trip distance.

If you take a long highway trip the warm up may not be a big issue, and air density will be a major factor. I mentioned my rule of thumb: driving 50 in winter is like 70 in summer. This is not exact, but you can figure it all out with your temperatures from the air density. These curves were posted here once way back.
Or get Wayne's Palm simulator to calculate this effect. It is not a minor effect!

If you are used to super mpg driving under 40 mph with lots of engine off time, then the battery temperature will be a major effect for you. For fast highway driving it won't matter as much.

So you must figure which factors are key for you! But ballpark I will throw our some guesses for numbers:

1 Winter Fuel: costs 6 to 12% (Note it can get worse for coldest part of winter.)

2 The warmup: costs 50% till warm

3 The air density: costs 0% at really slow speeds. May cost over 30% at high speeds. And this cost is for the whole trip!

4 Lubricants thick when cold: Synthetic oil can help. Overall maybe 3-4% till warmed up.

5 The car needs more heat to keep warm: Figure cost is in 2 so get and stay warm.

6 The battery limits when cold: Maybe costs 10% depending on the conditions.

7 In city driving may not be able to keep warm: Again cost may stay at 50% (see 2)

8 Roads wet etc: Costs 10% maybe for heavy rain, more for heavy snow.

9 Getting to stage IV (covered in the above) Remember 163 degrees, and this is why every Prius should have engine temperature readouts!

 

Yes.
Warm intake air mod results 5 to 10 % improvement.

Ken@Japan

 

Everyone needs to make their own tradoffs here just as in choosing a speed to travel in the summer.

I can tell you some things I do that are already spread around various posts here:

1 B-Quiet in the doors. Not just damping, but 2 layers of the foam absorbers. These also insulate, cutting the load on the heater and keeping the cabin warmer.

2 Block heater for 2-3 hours so I can start out warm, and not get too cold while waiting for the car to warm up. Also cuts engine warm up time significantly, getting me to stage IV typically over twice as fast.

3 Cabin heater on same power plug as the block heater. Timer brings this on about 15 to 30 min before I leave to warm the cabin and the battery a bit. Also makes the ice and snow just brush off the windows.

4 Engine insulation plus when really cold some radiator blockage too! Don't try this without temperature readouts! I try to get the car up over 150 F at least, and keep it under 195 deg F. I stop and adjust as required for the conditions. I leave the bottom of radator open for the inverter cooler.

5 When I have time I try to drive 5 or 10 mph slower than in the summer. But take highways more so I can stay over 40 mph when that doesn't keep engine warm.

6 I use an EV button, some, in winter but mostly to cut power early when I am arriving at a destination, so I don't waste more gas going slow trying to keep engine warm when it isn't needed any more.

7 Where I can adjust routes it is important to start fast and end slow because very slow driving and lots of traffic lights really delays the warm up.

8 I use temp readouts to adjust heater when possible so that I start with recirc mode and one of the two lowest fan speeds. Once engine is over 163 deg I take it out of recirc, and use manually selected faster speeds such that it stays above this temp. I use auto only when really warm enough ambient temps.

9 I check tires and keep my pressures at 42/40 as in summer.

10 I use Mobil 1 oil.

11 If warm enough for some engine off mode, I cut heat or go back to recirc so I am not cooling engine extra fast.

12 I look even harder for alternate routes when there is a traffic jam than in summer. With the cold you lose the advantage of the engine shutting off very quickly traveling 0-20 mph.

13 When I can use some engine off mode I use temperature as a major input on how much to let the engine stay off. So overall I keep engine on more in winter.
I benefit from EV button quite a bit in spring and fall, but not so much in winter except as mentioned above.

14 I combine trips more in winter, to minimize full warm up cycles.

15 Try to park in the sun if there is any. Makes a big difference when I am not home and can't plug in the block heater.

 

I recall hearing two mechanics talk about working on older cars. Some things that were mentioned were that older cars tended to run a lower rpms and they also ran much cooler. The effort to reduce emissions is what caused engines to have to be run hotter. The catalytic converter is a key part in cleaning up the emissions and the catalytic converter has to be hot to work properly.

So if we were not concerned about better protecting the environment we could run cooler and with better gas mileage.

 

actually it is very easy for one to determine themselves how temps affect mileage. i have a commute to work, distance less than 8 miles, mostly at 35 mph. its a commute that has minimal traffic hassles (mostly because of the weird hours i work) and is not far enough to dilute the effects of temperature on economy. best part is that this commute is usually done after the car has sat overnight or 8+ hours at work. makes for a consistent baseline to work with.

for anyone else who has a trip they make regularly, you can also see for yourself how temps affect mileage. now this doesnt take into effect the different gas formulations, strictly for temp affects only. my last two blogs center on this topic and discuss effects and methods of recording and graphing the info.

 

just one word.............batteries........batteries.......................batteries.........they hold a better charge in hot weather...........cold equals.......................very slow particals...no juice..........o boy

 

Holy thread resurrection.

Nope. The batteries are NOT the only reason. Besides whatever was discussed here, see:
Why does mileage drop in winter? — Autoblog Green
Cold Weather Vehicle Fuel Mileage – Why Winter Fuel Economy Drops – Fuel Mileage Drop in Vehicles
Car Talk
http://www.startribune.com/cars/11354696.html

 

batteries batteries and batteries......the warmer the weather the better the batteries hold a charge and discharge better.....cold weather equals bad battery charging and discharging

 

these "religious" resurrections always happens when a key "religious" leader passes away

 

One most commonly overlooked factor is that air density decreases as temperatures go up.. 10%+ going from 32F to 82F. This reduces air drag and max available power, both good for MPG.

Tire rolling resistance decreases as well due to higher pressure and flexier rubber; at least until pavement starts melting and getting sticky.

 

flexier rubber? does that not increase rolling resistance?

 

My church preaches oil viscosity as the fall of many a fuel economist.

 

here is some info: The Recumbent Bicycle and Human Powered Vehicle Information Center