[320dAsh]

I bought my car in December (7k on the clock) and was averaging 45mpg on my morning commute. This week,the mpg has been rising - 55.3mpg this morning for 34 min duration at an avg 20.0 mph. Can this really be down to the the air temp getting warmer or is it the engine bedding in more (it's now got 10k on the clock).
10 mpg difference between winter and spring seems a lot. Wonder what it's like as the mercury rises further?

[MattOz]

Yup, sounds about right.

[troggy1969]

I wonder if this is the change from winter to summer diesel. It's changed mid March for bulk deliveries and takes a while to work through. The anti-waxing additive makes fuel consumption worse on diesels

https://en.wikipedia.org/wiki/Winter_diesel_fuel

[stu14t]

It may be a combination of things like engine taking longer to reach optimum temp in winter and cold gearbox and trans fluids. I think more diesel is injected into a colder engine on startup too.

I know diesels are much better today but back in the day some diesel engines that needed to be working optimally instantly (like fire engines) used to have block heaters to keep everything warm and ready to go.

[HighlandPete]

Plus we have to add in the extra electrical load in the colder weather, including the high output HVAC cabin heater element.

[Soul_Glo]

If it's a diesel then you'll definitely notice an increase in warmer months.

[Ti Rich]

I would say the change is more like 5 mpg winter to summer. 10 is a bit high.

[creepy coupe]

In the cold months my 435d would sit on the motorway in 7th for about 3 miles. Which can't help.

[ahmedb]

10 mpg difference is a lot, a small amount of that will be the engine bedding in but the rest is entirely feasible to be down to weather/ambience.

Particularly if you used to depart/arrive in the dark and now you travel during daylight, these factors combined can save lots of fuel. I'm managing 52mpg in my 330d (when I try) compared to 40-42 in the winter.

[HighlandPete]

Originally Posted by Ti Rich

I would say the change is more like 5 mpg winter to summer. 10 is a bit high.

Same thinking, I've experienced about a 10% drop off in mpg for diesel motors, in an average winter. Can be more, if we have sub zero temperatures for sustained periods.

10mpg indicates more variables than just the ambient temperature changes.

[Rich9600]

Originally Posted by HighlandPete

Same thinking, I've experienced about a 10% drop off in mpg for diesel motors, in an average winter. Can be more, if we have sub zero temperatures for sustained periods.

10mpg indicates more variables than just the ambient temperature changes.

It's certainly not that huge.

Interesting, I've tracked my economy fir ages and looked back at my 3 BMW's. Uber geek...

I found my E90 320d SE exhibited less than a 10% winter drop on square summer tyres all year round.

My F30 330d MSport dropped about 5% in winter going from staggered summers to square winters.

My F31 340i MSport actually seemed more economical in winter... Though, it's clawing back a bit as the weather keeps improving.

I think with the rolling resistance change of a square winter vs staggered summer setup offsets some of this. Though, the 30d and 40i probably have been driven relatively more gently, even on the better days, in winter than the 20d was. All cars are sDrive...

Anecdotally; I got 38mpg out of the 340i visiting relatives on a nice day in the depths of winter on square winter rubber. Then went back at Easter on summers in heavy rain and only achieved 34mpg...

[HighlandPete]

Originally Posted by Rich9600

I think with the rolling resistance change of a square winter vs staggered summer setup offsets some of this. Though, the 30d and 40i probably have been driven relatively more gently, even on the better days, in winter than the 20d was. All cars are sDrive...

I used a staggered summer set on my E91 330d, still dropped back about 10% in winter. Then our average winter temperature up here is lower than those down south.

I found ambient temperature is the major factor in winter mpg figures. Made worse by 'shortish' 12 mile runs, where the diesel would not get to temperature in cold weather.

[insanecoder]

yup I am averaging 21-25 mpg near height of 90F summer
whereas I was averaging 28-32 in winter around 35F