Fitting the replacement for my blown turbo coincided with the onset of chilly weather here in the UK. My average fuel consumption has dropped from 40'ish to 36'ish. This is my first diesel car and I know they take a little while longer to warm up. I also don't go on very long runs (10-30 miles average). My E39 520i petrol predecessor would drop 1-2mpg in winter.

Coincidence and just normal for a diesel ?

Did the IDrive get reset to US gallons during the turbo replacement. It IS the default...

ABC

not a beemer, but the renault diesel im exchanging in a week or so does approx 42-43 when its cold, when it would normally do 45-47 otherwise. This is on a 50 mile run to work at an average speed of 45 mph (80 and 50 mix mostly)

cheers

steve

Can we now begin to draw a conclusion?

Is there some engineering or thermodynamic principle being indicated here?

ABC

My 530D gives 31.6 in summer 29.7 in winter.

Reasons,
1, Diesel engines take much longer to warm up than petrol.
2. More electrical gadgets switched on = more power drain.
3. Wet roads increase rolling resistance of tyres considerably
4. And...I think.....Colder fuel means the 'calorific value' is reduced, for a given amount of fuel by volume you get less energy out. So you have to use more for the same power. (I'm sure Znod or one of the other technical experts will put me right here!)

The biggest effect is with number 1. the others contribute in smaller ways but together account for higher consumption for all cars in winter.

All cars and enginetypes consumes more fuel when it's cold.

The cold starts is the key.

I cold engine has to do so much more work with all the stiff oils etc.

On the other hand once everything has warmed up properly shouldn't performance in a turbo petrol or diesel be better in very cold weather? You're essentially getting a denser charge and free intercooling.

I got 41.5 mpg from my 530d today over 230 mainly motorway miles, even including M1 Northbound traffic jams :)

Plus... higher density air = more aerodynamic resistance

the more power with cold air' is always an interesting one. Although its true, does anyone ever drive thier car on a cold day and think hey, this is faster today - dont think so - the effect must be pretty marginal.

the example I gave on my car was for a full 50 mile journey, each way, each day, over 2 years, so im pretty sure its not just the first few miles of warm up.... not over a 50 mile journey.

If the air charge is more dense, then there needs to be a proportionately bigger amount of fuel added ..... this makes sense no?

On the other hand, it might just be crappy engine mapping, overfuelling a bit to help with what would otherwise be a lumpy engine response.

In less than a week i will abe able to see how a 520d compares, as least in the cold

cheers

steve