Does COLD weather affect MPG???

Melensdad

Well-known member
Joined
Aug 30, 2009
Location
Rural N.W. Indiana, on the edge of the Lake Michig
TDI
2010 Jetta TDI, red w/Thunderbunny kit
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?
 

Lug_Nut

TDIClub Enthusiast, Pre-Forum Veteran Member
Joined
Jun 20, 1998
Location
Sterling, Massachusetts. USA
TDI
idi: 1988 Bolens DGT1700H, the other oil burner: 1967 Saab Sonett II two stroke
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.
 

coolbreeze

Veteran Member
Joined
Apr 18, 2008
Location
Troutman NC
TDI
2015 Golf TDI SE DSG - 2016 Tig SE for the wifey
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.

Melensdad said:
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?
 

Powder Hound

Top Post Dawg
Joined
Oct 25, 1999
Location
Under a Bridge, Crestview, FL, USA
TDI
'00 Golf 4dr White 5sp, '02 Jettachero 5sp, Wife's '03 NB Platinum Gray auto(!)
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.
 

coolbreeze

Veteran Member
Joined
Apr 18, 2008
Location
Troutman NC
TDI
2015 Golf TDI SE DSG - 2016 Tig SE for the wifey
.

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!:eek:
 

Rockwater

Veteran Member
Joined
Oct 8, 2009
Location
Denver, Colorado
TDI
2010 Jetta TDI Sportwagen, manual
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.
 

soldierguy

Veteran Member
Joined
Apr 13, 2004
Location
California
TDI
'15 Jetta TDI S DSG
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.
 
Last edited:

Brock_from_WI

Veteran Member
Joined
Sep 19, 2002
Location
Green Bay, WI USA
TDI
2003 wagon
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.
 

EddyKilowatt

Veteran Member
Joined
Mar 1, 2006
Location
Carmel Valley CA
TDI
2003 Golf GL 5M
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.
 

Lefty

Veteran Member
Joined
Oct 21, 2005
Location
Lazear,Colorado Population 60
TDI
Jetta, 2000,Green GLS
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.
 

jettawreck

Top Post Dawg
Joined
Aug 2, 2004
Location
Northern Minnesota-55744
TDI
2001 Jetta and 2003 Jetta
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.
 
Last edited:

juryblotter

Member
Joined
Dec 3, 2009
Location
Southwest Michigan
TDI
2010 Jetta Sportswagen
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.
 

RalphVa

Veteran Member
Joined
Oct 17, 2009
Location
Virginia
TDI
Jetta
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.
 

AlaskaAviator

Active member
Joined
Dec 3, 2009
Location
Alaska
TDI
06 Jetta 5 speed 96 Dodge Cummins 5 speed 92 Dodge Cummins 5 speed
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.
 

alnmike

Veteran Member
Joined
Feb 24, 2008
Location
Alaska
TDI
09
I travelled about 1k miles in the last week. Avg was 35mpg going between 55-65. In the summer that same trip going same speeds is 45 mpg. The temps dropped to -25F in Tok and Haines Junction.
Even driving 200 miles around here in 10+ weather I'm still getting 35. I know we run #1 diesel up here during winter, but I drove my car up here in early march when it was still around 0, and still got 45 mpg.
I added lightforces on the front of my car but am skepticle that they alone are responsible for a 25% hit to economy.
 

AlaskaAviator

Active member
Joined
Dec 3, 2009
Location
Alaska
TDI
06 Jetta 5 speed 96 Dodge Cummins 5 speed 92 Dodge Cummins 5 speed
Alnmike do you change to an allseason tire in the winter? On our Impala we go from 31 mpg in the summer to 27 in the winter and the only difference is the tires.
 

alnmike

Veteran Member
Joined
Feb 24, 2008
Location
Alaska
TDI
09
Haha, no, all season doesn't cut it up here. Winter calls for studded tires unless you've got 4 wheel drive and stick to busy streets. But when I drove up, I had the same set of studded tires waiting for me in Seattle.
 

boutmuet

Veteran Member
Joined
Jan 13, 2009
Location
Long Beach, CA
TDI
2015 BMW 328d
To be clear I don't think Californians get winter fuel is that correct? Its also my understanding that once your engine is warmed up it will run more efficiently with the cool air around it cooling it down without having any extra load on the engine to cool it down.
 

AlaskaAviator

Active member
Joined
Dec 3, 2009
Location
Alaska
TDI
06 Jetta 5 speed 96 Dodge Cummins 5 speed 92 Dodge Cummins 5 speed
Cold air is a lot better for the engine because it is so much more dense then hot air. That is why there is an intercooler to cool the air that the turbo just heated up. But with thicker air there would be a little more resistance just pushing the car through the air. When it got down to -30 or -35 if I would drive 50 55 the car would cool down in the 150 degree range, but if I stepped it up to 65 70 it would reach 190. At -40 it would not reach 190. Even at around -20 letting off the throttle down a hill the temp would fall right off. I beleive that anytime the engine runs much cooler, in the 30 degree or more range you probably lose some efficiency. I did not have anything on the front of the car. That would help a little on keeping the engine temp at 190. The good new is the heater still worked excellent even with the engine temp pegged on the bottom. As for allseason tires not cutting it here in Alaska I have lived here for 40 years and have never owned a set of studded tires yet. My wife does run them though. If people would buy the good winter compound tires and slow down just a little they would not need studs and we would not need to repave the glenn hiway every couple of years.
 

jettawreck

Top Post Dawg
Joined
Aug 2, 2004
Location
Northern Minnesota-55744
TDI
2001 Jetta and 2003 Jetta
AlaskaAviator said:
I just did a little cold weather trip and kept good records on mileage and temps.......

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.

A couple of interesting/noteworthy things there.
All of your figures are longish trips. Once the engine/tires/fluids/etc. are warmed up the cold then does not have as large an effect as some are seeing. But, drive 10, 20, or even 50 miles per cold start and you will see a much greater difference between warm/cold weather mileage. My commute is 40 miles each way. Most of the year my average is in the low to mid 50s. Cold weather average is mid to upper 40s. My fuel use chart going from fall to winter to spring looks like the Grand Tetons. :rolleyes:

Speed in the cold does make more difference than in warmer temps because of all the factors already mentioned, it all just multiplies. More work for the engine on fuel that has less energy.

I think you attribute way too much loss to "winterized" fuel. The energy content isn't 10% less, more like less than 5%, but the other cold weather factors contribute most to the mileage drop. I've run winter fuel in the warm weather and then the mileage drop is very little.

Good and accurate data for the trips. My wife and I were up to Denali by rail from Whittier this spring. Now I really want to drive it from home and take some time to enjoy the views.
 

jettawreck

Top Post Dawg
Joined
Aug 2, 2004
Location
Northern Minnesota-55744
TDI
2001 Jetta and 2003 Jetta
AlaskaAviator said:
Cold air is a lot better for the engine because it is so much more dense then hot air. That is why there is an intercooler to cool the air that the turbo just heated up. But with thicker air there would be a little more resistance just pushing the car through the air......
Yes, cold air, to a point, is good.
Cold/dense air carries more oxygen so for a given amount of fuel it burns leaner. (the reason why during cold/clear weather conditions we usually experience high barometric pressure-the air is heavy/dense) That's also why it is (or was before temp compensating fuel injection systems) so easy to burn up pistons/cylinders in 2 cycle engines in the cold. A fuel/air mixture that was set up for 20F becomes VERY lean @ -20F (carrys more 02) resulting in higher internal temps than the materials are capable of withstanding.
TDIs, and diesels in general, run very well in the cold air-as long as you can keep them fueled. In the VERY COLD the intercooler becomes self defeating and efficiency would be better w/IC covered to keep temps up in the range for best combustion in a compression ignition type system.
Cold/dense air's areodynamic loss is so minimal that you wouldn't be able to record it in a conventional vehicle with all all the other resistance factors.
 

jettawreck

Top Post Dawg
Joined
Aug 2, 2004
Location
Northern Minnesota-55744
TDI
2001 Jetta and 2003 Jetta
boutmuet said:
To be clear I don't think Californians get winter fuel is that correct? Its also my understanding that once your engine is warmed up it will run more efficiently with the cool air around it cooling it down without having any extra load on the engine to cool it down.
I'm sure some areas get "winterized" fuel. The whole state isn't nice and warm all the time.;)
There is no "extra load" on an engine to cool it, other than the electrical load from the alternator to run the cooling fan, if it got warm enough to need it. Waterpump runs all the time, its just a matter of the t-stat opening or closing. When the coolant, intake, and fuel temps get high the ECU makes adjustments which lower power output.
The TDI's computer (even the older ALHs, like mine) makes adjustments in injection timing, boost, and fueling depending on a ton of variables. Barometric pressure, load, coolant temp, ambient (intake) temp, fuel temp, etc, etc...They're very complex, one expert compared it to a mini weather station on board.
 

Osage Orange

Veteran Member
Joined
Nov 21, 2008
Location
Lewistown IL
TDI
2009 Jetta *sold*
Melensdad said:
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?
If you do much idling such as a 3-5 minute preheat, youll see your score drop a couple points. These diesels are still pretty efficent at idle though, my Vette Z06 loses mpg points instantly at idle.
 

jbright

Veteran Member
Joined
Sep 10, 2009
Location
Indianapolis
TDI
2009 Jetta DSG
Well, it finally got cold enough here to test my '09. 12 F this morning. The car started right up. I had a 30 mile drive on freeway and suburban 4 lane sprawl (to a TDI guru but that's another story) and I got 37 mpg. Driving home, same route, etc. I got 46 mpg. This is my first tank of winterized fuel. I am very pleased with how it performed in the cold. Next, I need some snow, and then some sub zero temps. Sure I'll get those soon enough!
 

AlaskaAviator

Active member
Joined
Dec 3, 2009
Location
Alaska
TDI
06 Jetta 5 speed 96 Dodge Cummins 5 speed 92 Dodge Cummins 5 speed
jettawreck said:
Yes, cold air, to a point, is good.
Cold/dense air carries more oxygen so for a given amount of fuel it burns leaner. (the reason why during cold/clear weather conditions we usually experience high barometric pressure-the air is heavy/dense) That's also why it is (or was before temp compensating fuel injection systems) so easy to burn up pistons/cylinders in 2 cycle engines in the cold. A fuel/air mixture that was set up for 20F becomes VERY lean @ -20F (carrys more 02) resulting in higher internal temps than the materials are capable of withstanding.
TDIs, and diesels in general, run very well in the cold air-as long as you can keep them fueled. In the VERY COLD the intercooler becomes self defeating and efficiency would be better w/IC covered to keep temps up in the range for best combustion in a compression ignition type system.
Cold/dense air's areodynamic loss is so minimal that you wouldn't be able to record it in a conventional vehicle with all all the other resistance factors.
Actually temperature has no effect on barometric pressure, does have effect on density altitude. The combination of cold air and high barometric pressure can have huge effect on air density. We left the car parked and idling for 5 hours at -40 in Fort Nelson and was very surprised at how well it rolled, I mean it didn't feel any different then when we parked it that night. Sometimes the tires develop flat spots and it takes a while to get them round again. I parked my 92 dodge Cummins once over night at about -50 and the next morning had to use low range to move it. Have since switched to Amsoil gear oil and eliminated that problem. Then one time was driving the same pickup and at the end of a several mile strait stretch of road came to a turn and almost went in the ditch as the power steering fluid was so thick. And then had to stop a few miles further and the brake fluid was so thick barley got stopped. It was -55 there. Any way this TDI passed the cold weather test with flying colors, can't get over how good the heater worked.
 

Turbosprezarka

Veteran Member
Joined
Feb 1, 2006
Location
New England
TDI
2001 Jetta TDI (ALH) GLS, 5sp, Tornado Red, 301,593 miles, SOLD
Even if #1 fuel is mixed with #2 fuel to help winterize it, I'm not so sure #1 fuel really has a LOWER energy content.

I have a book that lists calculated gross energy contents of fuel oils #1-6. #1 is listed as 19,670 to 19,860 Btu/lb. #2 is listed as 19,170 to 19,750 Btu/lb.

The cold temperatures affect the combustion. If the combustion occurs at a certain temperature, well then more energy has to be added to bring the ambient temperature (fuel, air), up to the combustion temperature. If you're driving around when it's 10F outside, you're engine is continuously losing more heat than if its 80F outside

The rolling resistance definitely increases. I drive to work each day on a route that sends me down a large hill (1.4 miles). There is a sign right at the top, and I've been doing some tests every morning to see how far I can roll without going below a 40 mph, starting at that sign, and at 55 mph. When it is warmer I absolutely go farther, a good 0.1-0.2 miles or so, then when it's been really cold lately.

There are so many factors involved, it'd be hard to pinpoint the exact reasoning. You are best off checking your mileage when you fill up each time, and comparing that way. a 9mpg difference seems like a lot, it could be just a fluke tank. Everyone has tanks that are just off every once in a blue moon.
 

AlaskaAviator

Active member
Joined
Dec 3, 2009
Location
Alaska
TDI
06 Jetta 5 speed 96 Dodge Cummins 5 speed 92 Dodge Cummins 5 speed
I will know a lot more in a few weeks but I am convinced that speed is the biggest factor in mileage regardless of temperature.
 

jettawreck

Top Post Dawg
Joined
Aug 2, 2004
Location
Northern Minnesota-55744
TDI
2001 Jetta and 2003 Jetta
Turbosprężarka said:
Even if #1 fuel is mixed with #2 fuel to help winterize it, I'm not so sure #1 fuel really has a LOWER energy content.

I have a book that lists calculated gross energy contents of fuel oils #1-6. #1 is listed as 19,670 to 19,860 Btu/lb. #2 is listed as 19,170 to 19,750 Btu/lb.

The cold temperatures affect the combustion. If the combustion occurs at a certain temperature, well then more energy has to be added to bring the ambient temperature (fuel, air), up to the combustion temperature. If you're driving around when it's 10F outside, you're engine is continuously losing more heat than if its 80F outside

The rolling resistance definitely increases. I drive to work each day on a route that sends me down a large hill (1.4 miles). There is a sign right at the top, and I've been doing some tests every morning to see how far I can roll without going below a 40 mph, starting at that sign, and at 55 mph. When it is warmer I absolutely go farther, a good 0.1-0.2 miles or so, then when it's been really cold lately.

There are so many factors involved, it'd be hard to pinpoint the exact reasoning. You are best off checking your mileage when you fill up each time, and comparing that way. a 9mpg difference seems like a lot, it could be just a fluke tank. Everyone has tanks that are just off every once in a blue moon.
Agree w/most of the above, except:
"Even if #1 fuel is mixed with #2 fuel to help winterize it, I'm not so sure #1 fuel really has a LOWER energy content."
I think the reference material has an error.
Pretty sure that #1 fuel has lower energy/BTU content. Produces less heat in a furnace, too. Any experienced diesel owner will avoid it except when needed. Costs more and delivers less MPG. Most bulk fuel dealers in most parts of the country don't "blend" #1/#2 as much as they use additives now, although for those of us in the cold country many places (truck stops, bulk dealers, etc) still offer straight #1 for use in "blending" your own or using as is when temps go crazy cold. Lots of construction/logging contractors use this for equipment that will sit outside unheated for periods of time and need to take no chance that the fuel will gell up. I put it in my tractor that sits most of the winter.
To drop 10 mpg from my warm weather average isn't uncommon when the temps go down below (or way below) -0F.
But it is a combination of many things, not just this or that one. Short trips (less than 50 miles) really have an effect. Takes a long time to warm things up when its really cold, and some things never really do.;)
 
Top