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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old December 11th, 2009, 04:37   #1
Melensdad
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Question Does COLD weather affect MPG???

I drive 90 to 100 miles on a typical day. Most of my driving is rural 2 lane road and suburban 4 lane roads. Since buying my 2010 Jetta TDI the average fuel economy for roughly 1800 miles driven has been right about 39.5mpg. That all came to a screeching halt when our temperatures dropped to near ZERO.

For the past two days we've had temps that have been between 2 and 10 degrees (F). Fuel economy on the same roads has dropped to roughly 30 to 32 mpg based on the MFI display. GRANTED there has been some LIGHT snow on the road. And GRANTED snow causes some of the traffic to clog a bit more. But these are the same roads, same travel times, etc that I commute every day.

I can understand losing a few MPG to some snow/traffic conditions. But to drop from 39+mpg down to roughly 31mpg seems unreasonable to attribute to the LIGHT snow conditions.

Thoughts?
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Old December 11th, 2009, 04:45   #2
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Fuel.
#2 (Diesel) fuel is blended with #1 fuel to lower the temperature at which it starts to 'freeze' or gel into a thick syrupy mess that can't be pumped. This 'winterized' diesel has lower BTU content attributable to the #1 fuel.
The choice is lower mpg with a winterized liquid fuel, or walk because the car won't run on summer fuel.
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Old December 11th, 2009, 04:50   #3
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Lots of posts and threads on winter mpg, but to put it in a nutshell you have winter fuel hitting you for a few mpg because of the lower energy content from additive to protect against jelling. Lower temps are hitting you for a few as the CBEA engine seems to be more sensitive to cold starts. I would also suspect that lower rolling efficiency from snow, salt and ice will hit you for a few. All part of cold weather mpg and nothing to worry about. Your mpg will be even better this spring when summer d2 is back and warm weather is here. Do some searches for winter mpg if you want a better explanation. Enjoy your car and welcome to the club.

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Originally Posted by Melensdad
I drive 90 to 100 miles on a typical day. Most of my driving is rural 2 lane road and suburban 4 lane roads. Since buying my 2010 Jetta TDI the average fuel economy for roughly 1800 miles driven has been right about 39.5mpg. That all came to a screeching halt when our temperatures dropped to near ZERO.

For the past two days we've had temps that have been between 2 and 10 degrees (F). Fuel economy on the same roads has dropped to roughly 30 to 32 mpg based on the MFI display. GRANTED there has been some LIGHT snow on the road. And GRANTED snow causes some of the traffic to clog a bit more. But these are the same roads, same travel times, etc that I commute every day.

I can understand losing a few MPG to some snow/traffic conditions. But to drop from 39+mpg down to roughly 31mpg seems unreasonable to attribute to the LIGHT snow conditions.

Thoughts?
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Old December 11th, 2009, 04:55   #4
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The cold does 2 things: 1) makes it more difficult for the engine to maintain temperature, and 2) winterizing fuel seems to drop its heating value.

For point 1, our diesels are pretty thermodynamically efficient, which means more of the heat energy generated by the fuel gets turned into mechanical energy used to drive the car. Even the turbocharger helps by extracting a little more heat from the exhaust and using it to compress the incoming air charge.

But, an engine will not run as efficiently when it is cold. The compression won't be as good, the piston rings won't seal quite as good, the oil is more viscous and will suck more energy to pump it, the alternator has to generate more juice to run heaters, lights, as well as more juice to recharge the battery, and generally, things just don't go as well. Even the coolant will take more energy to pump it.

Used to be that winterized fuel was thinned with kerosene, but most refineries claim they do it chemically now. I'm not sure that is 100% of the treatment, but I'm not going to run to a chemical lab with a fuel sample, either.

But with increased rolling resistance in cold, even more in snow (it sucks a lot more energy than you'd think, even if it is 'light' snow), and the struggles to keep warm, less energy out of the fuel to boot, it is no mystery why fuel mileage drops a lot in the winter.

Mine dropped only marginally when I lived in Phoenix. Of course there, the fuel isn't winterized, and a 30 degree low temp makes everyone wonder if the sky is going to fall in next. When I move to NH, I experienced winter fuel mileage lower than anything I've ever experienced anywhere else.

So yeah, it isn't fun to see that happen. I compensate by staying indoors and have wheel polishing, headlight protector, and LED light development winter projects in mind. I also bake cookies too, but I have to curtail that one as it makes my skiing and bicycle riding (come spring) suffer.
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Old December 11th, 2009, 04:59   #5
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.

I also bake cookies too, but I have to curtail that one
as it makes my skiing and bicycle riding (come spring) suffer.[/quote]

I find it helps on the down hills though!
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Old December 11th, 2009, 08:44   #6
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Another factor pointed out in other threads related to this topic is tire pressure which drops with temperature thus causing lower mpgs. Check and adjust if necessary.
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Old December 11th, 2009, 12:44   #7
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Two other things to consider:

1. When vehicles are cold, they typically run rich to help get the engine up to normal operating temperature. They ignore what the sensors are telling them, and dump extra fuel in. In extremely cold weather, it simply takes longer for the vehicle to reach operating temperature, so you've got a longer period of time that your engine is getting extra fuel pumped into it.

2. Percentages and perceptions. Accounting for all the factors mentioned above and using your example, you're getting about 80% of the fuel economy that you used to get during better weather. Let's also assume that nothing is actually wrong with the vehicle. If you were in a vehicle that originally got 20 mpg under your conditions and it suffered the same percentage of loss, you'd be getting about 16 mpg, a 4 mpg loss. A noticeable difference, but losing 4 mpg doesn't seem like a huge drop in economy. Your vehicle gets nearly double, at almost 40 mpg. Applying the same percentage of fuel economy loss, your mpg loss looks huge, but expressed as a percentage, it's the same as any other vehicle that is getting about 80% of it's normal fuel economy. I don't know what you used to drive and what the percentage of fuel economy it lost in the winter, and I don't know if your loss is typical (I try to stay out of the cold states) but it's at least something to consider.
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Old December 11th, 2009, 13:32   #8
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If you have a scan gauge look at the engine temp. My guess is your running cooler than you normally do. I find that when I am below 175F the mileage is off. For me this means I have to do 100% block on the front to get up to and stay at 180+F. So you might simply try blocking part of the front to keep the engine warmer. This is more noticeable in rural driving conditions where your not going as fast and not stopping and starting a lot.

As others have mentioned the primary cause is the fuel.
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Old December 11th, 2009, 14:41   #9
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Well I filled up the tank this morning, drove about 120 miles today, got back up to about 38mpg.

I'm guessing most of my issue was fuel, some was probably snow, some was probably cold.
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Old December 11th, 2009, 15:40   #10
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I have a long straight offramp where I can routinely do an informal coastdown test in neutral to check rolling resistance. It comes at the end of my 30-mile commute so operating temperatures are pretty stabilized, and I do it at low-ish speeds (45 -> 35 mph) to keep air resistance effects small.

I can report that RR goes up noticeably, like like several percent, in cold weather. I reckon there are a bunch of factors in play, like thicker lubricants, lower tire pressure, more hysteresis in the tire rubber perhaps (?), denser air causing more aero drag... bottom line, aside from the decrease in fuel energy, it is definitely harder to push the car down the road in sub-freezing temperatures.
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Old December 12th, 2009, 13:35   #11
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I went from 49.25-49.75 the last few tanks before the cold came in to 44.02 this last tank. I let my car sit and warm for about 5 minutes(takes fuel) I use the heater constantly(takes more fuel) As mentioned the tires are cold,been -6 for 3 days so tires never get up to temp.

And did I mention,it has been cold.
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Old December 14th, 2009, 07:31   #12
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Does COLD weather affect MPG???
How can it NOT, is a better question.
In addition to some of the already mentioned.
Rolling resistance from your tires is much higher when its cold. Just try pushing the car from a level parked spot when its below 0F compared to warm. Sure they warm up some after being driven a distance, but it takes more fuel for a while. Tire air pressure also drops as the ambient temp does, as mentiomed.
Frictional or perhaps fluid/grease resistance. Tranny oils, differential oils, wheel bearing grease, axle/cv joint grease, etc. they all thicken up a bunch when it gets really cold. Takes more energy to turn all this stuff. A lot of them never really warm up in the COLD. On my commute home there is a grade downhill into a little town where the speed limit goes from 55 to 30. In warm weather some early coasting and a little braking will get you by, but when its below 0 you cannt even maintain speed down the grade in coast mode.
Lot of things working against you in the cold, even if the road is clear-which it usually isn't.
I normally run an average of around 52/53 MPG in above freezing temps. Below freezing my average is 48. When its -0/-30F I'm glad to get "only" 44 mpg. My mileage graph looks like a profile of the Rocky Mountains in spring and fall.
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Old December 14th, 2009, 18:01   #13
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I just purchased a 2010 Jetta Sportwagen a few weeks ago. The fuel tank was full when I drove off the lot and the temp during the time I drove that tank were between 40f and 60f. I averaged 39.5mpg. The next tank averaged 35mpg. the temp were between 8 and 30 degrees. I'm assuming it has something to do with the weather/fuel. I just don't know when they changed to the winter blend. Possibly the fuel in the car when I drove it off the lot had been in it since the weather was warmer. I'm running my third tank full and keeping track. The temp are between 20 and 30.
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Old December 15th, 2009, 16:03   #14
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Sure does.

This morning, I got in the TDI, and the fuel economy meter still showed the 36 mpg from my wife's round trip early this morning to the gym.

I improved the mileage to 39 before I got to the bowling alley. The outside temperature was warming some.

On the way back with outside temperature all the way up to the 50s, I started with a blank trip mileage showing. It showed 47 mpg by the time I got home.

On our recent trip to Columbus, OH, the trip out recorded 44 mpg on the trip computer, 43.5 by tank, with outside temps in the 50s.

On the way back, temps had dropped to the 30s. The trip computer showed 42 mpg, only 40 by tank because of one really cold start and not far enough to warm it up.
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Old December 15th, 2009, 20:05   #15
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I just did a little cold weather trip and kept good records on mileage and temps.
Left Dallas and drove 446 miles and used 11.2 gal - 40.1mpg outside temps ran about 39 degrees. Interstate driving about 70 mph

Left Whitecourt Alberta drove to Dawson Creek BC 261 miles and used 7.4 gal - 35.2 mpg about the same speed maybe a little slower, temps where between -26 and -44

Left Toad River drove to Whitehorse 483 miles and used 13.7 gal - 35.2 mpg
averaged about the same speed again, drove slow, around 55 at first and after it warmed up was running 70 and 80 for about 150 miles. Temps ranged from -46 to +10 near Whitehorse.

On the last leg of the trip drove from Whitehorse to Tok Alaska and got 48 mpg. Temps all above 0 but drove under 62 the whole way.

So I don't think cold effects your mileage as much as speed. I only lost 5 mpg going from +39 to -40. But keep in mind the fuel I got in Dallas was #2 and the fuel in Canada was #1. In my 5.9 Cummins the fuel makes about 10% difference in mileage, and would assume about the same in the Jetta.
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