[JaySTee]

Only on my second tank since I got the car but since it turned really cold I am struggling to get the same mpg (320D). Gone from 41 ish to 37 ish.

Why does the cold affect MPG is it just that the engine takes longer to warm up?

I only have a shortish drive to work of around 8 miles / 25 mins and can be a bit stop start so I guess it is propbably the not warming up fully thing. Shame they don't have a temp gauge, I need to set the OBC to give the temp display I guess and check it is getting to temp as it should...

[jimmylad]

it's a strange one...........

It must be because of the whole engine / transmission taking longer to warm but you would think that the amount of cold air about ( colder denser air = more power but have to say dont notice any difference) would make the car more powerful thus reducing consumption offsetting the frictional losses.

[jimmylad]

my consumption did drop with the cold weather but has gone up due to removing dpf so I'm none the wiser.

Off to Crewe from Derby so gonna see what a 100 mile trip returns........

I do find that trying to get max mpg at 50mph is a waste of time as the engine isn't labouring as such but doing 60 - 65 keeps the instant mpg needle above 50mpg a lot more & is in more of a sweet spot.

Anticipating stopping has a big effect.

[HighlandPete]

Lower MPG, absolutely.

Forget the power/denser air issue when starting from cold. We have two major issues here, parasitic losses, due to cold components, massive electrical load, etc., plus diesel combustion doesn't work efficiently unless the cylinder walls/chambers are up to temperature, basically the flame is being partially quenched. Also remember engine oil temperature can take about 15 miles to get into the ideal range from sub zero temperatures.

Warm up times are dramatically increased by cold weather, don't be fooled by a bit of warm air either, as the HVAC has an electrical element boosting the cabin heating, also putting an electrical demand that's using around 2 bhp (Net).


I've been watching the old 'swingometer' at a specific part of my trips, where I can easily replicate the results. If I select cruise and 60mph, on a warm day (with a hot engine and transmission) I see just over the 50mpg line. Get changes in the weather, temperature, electrical load, or distance driven and the figure can be as wide apart as just over 30mpg to 50mpg. After just 6 miles, at sub zero temperatures it will be about 35mpg. After running about 20 miles, but still sub zero, the figure will be about 45mpg. Now that is one of the easiest, steady speed checks. Now think how all the other factors change mpg in winter time, particularly on short 'cold start' trips.

HighlandPete

[JaySTee]

jimmylad,

Yeah that did cross my mind...


HighlandPete,

Thanks for the comprehensive explanatioin. I didn't realise the heater had an electric element, so that in conjunction with lights and rear window heater will make a noticeable difference I guess, along side the cold air temps and the short journey time.

[HighlandPete]

I noticed just yesterday that driving off from cold (-11 degrees) how laboured the car felt for the first 5 miles. The whole car seemed 'half set', and of course it holds off 6th gear until there is some heat in the engine/exhaust (cats and emissions) and the transmission for direct lock-up.

We have large capacity alternators and batteries these days, it is not just for the starting, but the demands over the first few miles. Even the auxillary (PTC) heater is only able to give maximum capacity if the load on the electrical system allows it. So still not over doing it with electrical capacity in real terms.

I reckon that I have a 10% mpg penalty in winter, over summer consumption. (From the spreadsheet). The more economical a diesel is, the more that shows in lost mpg. 3 - 4 mpg in the 330d, 5 - 6 mpg in a 2.0 litre diesel.

HighlandPete

[m1bjr]

Have you rechecked your tyre pressures?

They may be several PSI lower at subzero if you last set them at say 15c

[briers]

Haven't noticed much difference myself

[jimmylad]

just been to Crewe from Derby. 100 mile round trip. minus 7 in Derby & Minus 3 in Crewe. Car did take a good 10 miles for everything to warm up & the car didn't want to drop into 6th like it does when its warmed up.

Averaged 49.5 on the trip but what is interesting is that if you reset the journey computer & hover around 65mph the mpg is bloody impressive. See following pics.

[JaySTee]

Originally Posted by m1bjr

Have you rechecked your tyre pressures?

They may be several PSI lower at subzero if you last set them at say 15c

Yep checked recently.

Briers - I think in my case it is compounded by the fact that I mostly do relatively short runs as in my first post. If you are doing longer trips you would probably not notice such a difference...

[briers]

Originally Posted by JaySTee

Yep checked recently.

Briers - I think in my case it is compounded by the fact that I mostly do relatively short runs as in my first post. If you are doing longer trips you would probably not notice such a difference...

I haven't really monitored the MPG much recently but i was going to say the short trips probably aren't helping. Most of mine are > 30mins and mainly A Roads.

[mobileadam]

I only really do shorter trips and I've seen my average drop from 42mpg to 37 ish

[HighlandPete]

Just as an example of how the short trip can impinge on mpg, both for the 540i & 330d. Figures by the time I get to the next village road junction, (3-miles). BTW, I could/can mimic conditions/speed quite easily:

540i

Cold start around 0°C; OBC reads 17-18 mpg.
Cold start approx’ 10°C; OBC reads 20-21 mpg.
Hot start at approx, 10°C; OBC reads 26-27 mpg.

330d:

Cold start around 0°C; OBC reads 24-25 mpg.
Cold start approx’ 10°C; OBC reads 29-30 mpg.
Hot start at approx, 10°C; OBC reads 37-38 mpg.

Now think about how many miles it takes to get to similar mpg, from the cold starts. The 330d takes about 10-15 miles to reach 35mpg from a 10°C cold start, typically about 50 miles to get to 40mpg.

From sub zero temperatures, even after 60 miles, OBC's typically showing 37.1 mpg.

HighlandPete

[Jeff123]

I am struggling to get more than c36mpg at present on short(ish) trips, from an auto. Last winter in similar conditions, I had a 250 mile trip each week; off at 05:30, so not hanging around. I averaged around 47 on that trip - so distance does have a major affect.

[E92westy]

Mines definitely worse.
Was 41mpg now 36 - 37mpg since the cold snap.

[JaySTee]

Well seems i'm about normal then which is always good to know

I was never obsessed by fuel consumption until I started getting cars with trip comps. I can and do turn the display off but then on the BMW's you still get the swingometer glaring at you...

[Shadow]

Isn't winter grade fuel slightly lower in calorific value, as the formulation is different to cope with the cold? Don't know how much difference that would make...

[Carrera RSR]

Cold denser air = more drag to push the vehicle through.