mpg -hot and cold

[weldore]

its not really a problem but more of an annoyance.
ive noticed over the few years ive had the car that the mpg computer will give me some varying results depending on the temp outside.
if its been warm the car will easily do 50mpg but during winter the car will go right down to under 40mpg.last winter it said i was doing 36mpg mostly.
no amount of lightfootedness will stop it dropping like a brick after resetting the comp under 10 deg c but over that it will give back decent results up in to the late 40's and early 50's
i just wondered if they were 'just like that' or i had a problem somewhere ?

any info appreciated guys

[weldore]

as an after thought..do you think that having the blower on would effect the mpg as much as this?

[AlexB]

Under the rated temperature the engine consumes a lot. It is all about warming up. Modern engines are too efficient to warm up quickly.

[weldore]

so this is to be expected alex ?

[AlexB]

Yes. If you only do long journeys, then the difference will be small, because the engine stays hot all the time. On short journeys the difference can be significant. The other two contributors are the traffic, which is slower and denser in winter, and the tyre pressures, which many of us do not check when the temperature drops.. What engine are we talking about?

[weldore]

its a 1.9 DCI
it seems a bit excessive though from 50 down to under 40 thats over 20% drop and the journey's i do are usually the same all year round....i blame the wife !!!

[morganjake1544]

Don't forget in winter you use the lights, wipers and heated rear window/heated mirrors a lot more as well as the fan on higher settings to clear windscreen. It all adds up.
Having said that mine only drops maybe 4mpg (59 down to 55mpg).

[davelowe]

I have no better explanation other than has already been said. Though, apart from the higher load on the alternator, why would the temperature make such a difference - all other things being equal? Alex's reply makes the better sense.

Of course, colder air is more dense, this requiring more fuel to compensate - but your situation is not like driving up and down a 10,000 foot mountain.

[weldore]

its an odd one aint it...it does just seem to be sub 10c it all starts to throw a wobbly.never mind,its a bit warmer now