In the summer I get 600 miles to a tank, in the winter it drops to 530 ish. I don’t run the air con, so that isn’t a factor.

Short Journeys on the choke?

My petrol does better MPG and goes better in the winter!

ours does, diesels don't like moisture either apparently

According to my on board computer my fuel economy had dropped by 10mpg in my Focus. I put this down to being 9000 miles over the service interval schedule. Booked in for a service so that should sort that out.

Technically they should be more economical due to the colder air, but factor in heater use, lights, heated windows, windscreen wipers etc it drops off.

different grade of diesel perhaps. Winter fuel is less dense.

I've noticed less smoke from the rear in the colder months when I hit the LOUD pedal hard. I've assumed the engine prefers to cold and haven't really noticed a variance in MPG – but then my driving trips are vary varied (no daily commute).

I'm glad that it is not just me then. I only do 8 miles each wway so it never gets warm (and no I can't cycle to work before anybody tells me off)

diesels don't like moisture either apparently

That may explain why it is as rough as a bear's arse too.

My petrol turbo engine has dropped about 10% in the winter, I assume because I am using lights, heated seats etc. also takes longer for the engine to warm up.
The fuel economy was almost acceptable in the summer, but its now mid 20's, so looks like a new car might be on the cards.

Doug

Just realised the topic was for diesel engines, same applies I guess

Yes ive noticed the same thing since every winter, had the diesel 3 years now.

Short Journeys on the choke?

Are we back in 1972!!!

yeah i noticed this.

I think they put stuff in the diesel to stop it turning to sludge when it gets cold. The engine takes 10-20 minutes to heat up too so that can't help.

I read a similar post somewhere from a VW driver who was doing an all day trip abroad that started in warm weather and ended in cold weather (day/night) and as the temp dropped so did the fuel economy.
I think I read that its something to do with the warmer air reaching the combustion temperature in the cylinder earlier/quicker than cold air – but it could have been something else.
I doubt that heater use would make any difference as the heat comes from the engine coolant and is not created electrically.
I also doubt that lights would make much/any difference as the engine is turning the alternator all the time unlike AC, where the compressor only tuns when the AC is switched on (or that's the case on my car).

But whatever the reason it does seem that fuel consumption goes up in the winter.

…and my next question. Why is the carpet in the rear passenger side footwell damp?

…and my next question. Why is the carpet in the rear passenger side footwell damp?

It's leaking?

Cooler air is denser, so in theory that means more efficiency/power – hence why most modern turbos have an intercooler between turbo and inlet to cool the air that has become heated as it passes through the turbo.

But in reality winter always hits fuel consumption. More use of lights, longer time to warm up, even maybe more congestion on local roads due to poor weather. Some people find that the nature of their driving changes, more shorter trips instead of longer runs.

Yeah, I get this too. I think it's a combination of many factors.

On my commute there seems to be more queues as people seem to drive slower once the nights draw in & it's wet & it's a bit misty etc.

I have noticed that the best weather for the car seems to be a cold, crisp, dry morning when the engine seems to have a little more whizz. It also seems to dislike it when the air is very damp, which it often is throughout winter in this country.

You also seem to get more windy days (particularly lately) and that can really bring down your fuel economy. Annoyingly if you are doing a there & back journey, your car won't seem to be extra-eficient on the way back!

Also takes more time for the engine to warm up & you are more likely to have the headlights on, heater, heater mirrors, screen etc. so on an 8 mile commute you probably have the alternator chugging away for a lot of it.

Not really, been using V Power and Excelium which seems to be fine. Disco TDi.

sharkbait – when you use more electrical gadgets the resistance in the alternator gets higher (to produce more amps) this is why if you turn your lights on at idle the revs drop a bit.

Oh, and whoever ridiculed the choke idea, just because its not manual doesnt mean your car doesnt have a choke 😉 Theyre just automatic these days.

Why is the carpet in the rear passenger side footwell damp

If it's a Passat (or perhaps other VWs too) it's probably water leaking in because the drain in the engine bay is blocked (leaves etc). A known fault – mine caused the electrics to go haywire too (there's a tray under the passenger seat that fills up). To clean out the drain you have to take out the battery and other bits and bobs. If you think that's what it is get it sorted asap.

you probably have the alternator chugging away for a lot of it.

Surely the alternator is always producing power? It doesn't disconnect from the engine when it's not required (almost never) so it would make very little, if any, difference to the draw on the engine.
I could be wrong though.

when you use more electrical gadgets the resistance in the alternator gets higher (to produce more amps) this is why if you turn your lights on at idle the revs drop a bit

…….ahhh….. I am 😳

If it's a Passat (or perhaps other VWs too) it's probably water leaking in because the drain in the engine bay is blocked …

Its a Vectra. I think that they suffer from something similar so I've just rodded out the drain under the windscreen wiper mechanism cover.

I was thinking exact same thing this morning, mpg dropped from 44-45mpg down to 38-39 on my 85 mile commute to work in golf tdi.

It was super foggy so guessing it's the moisture rather than the cold that does it? As above, I'd have thought cold air == better.

My Volvo started getting wet pillars on the inside of the windscreen. Turned out to be the drains from the sunshine roof that get clogged with crap and then leak at an elbow where they exit the bodywork.
Removed the trim inside, disconnected the drain pipes, cleaned out and refitted the pipes with some silicon sealant for good measure and both leaks stopped.

MFL has hit the nail on the head. Its related to air density, which as we know, is an air tempareture related matter.

And as others have mentioned, increased use of more electrical systems will drop the MPG a little bit, but main culprit is air temp.

You see it on vehicles when they are testing in places like Finland, both petrol and diesel MPG drops off in cold temps.

Wet carpet: as mentioned above, is a leak. A friend has a VW passat, said some VWs had a fault with the rear passenger window dropping occassionaly. Obviously model and YoM related.

Also check the door to bodyside seals. There may be water drain holes at the botton of the door, and water may be getting through the door (past the glass and weather strip interface for example) and into the car.
🙂

Solo.

it's a secret conspiracy of brown and darling's to make you buy more fuel so you can fund their £178Bn credit card habbit

but factor in heater use, lights, heated windows, windscreen wipers etc it drops off.

the heater uses waste heat from the engine, apart from the juice to drive the fan. I'd be surprised if all those together were more than a kilowatt, which is 1.3bhp, so what 2% of the power of a small engine ?

Cooler air is denser, so in theory that means more efficiency/power

Theoretically also harder to push a vehicle through as well 🙂

…and my next question. Why is the carpet in the rear passenger side footwell damp?

You emptied your shoes after you complained to the garage that their diesel was inefficient?

1. Different grade of fuel used in winter to stop the fuel waxing.
2. More electrical consumers used, therefor higher load on the engine.
3. Longer warm-up cycle due to lower abient temprature.

Normally enough to give a circa 10-15% increase in consumption of diesel.

noticed this recently, too. i had put it down to it needing a service.

Another thread talking about the effects of electrical draw on fuel consumption here.
They basically decided it was barely worth worrying about, although heated screens may have a bigger effect. My car automatically switches off the heated screen so there may be something in that one.

Friction – i.e. the power needed just to turn the engine round even without driving anything increases with decreasing temperature. Made worse by the engine being slower to warm up both oil & coolant. Mixing of the the fuel into the air is poorer at low air & combustion chamber temps – more fuel lost into the oil & if the oil does not get hot enough it does not evaporate back out. For petrol engines the power needed to pump the air into the engine at part throttle increases as the more dense colder air needs more throttling to lower inlet manifold pressure. Electrical loads don't help but will probably be difficult to measure fuel wise. Power steering oil is thicker & harder to pump around – same for gearbox & axle – the list goes on…

the air density thing is nonsense. Whilst denser air means more air in the engine, and a touch more power at full throttle, it won't have an effect on fuel consumption except for the fact that compression losses may be higher (more air in there = more air to compress) on a non-throttled engine (ie a diesel). The fact there is more air in there won't make the engine use more fuel to 'use up' the air, in a diesel there's just more unused air left over.

99% of the change is going to be shorter journeys, longer warm-up time, and more fuel used due to the lower-non-warmed up temperatures. My (old) mondeo is an absolute dog after its been standing for a couple of weeks in this weather, takes 2 or 3 hits on the glow plugs to get it starting well

sharkbait – Member

Surely the alternator is always producing power? It doesn't disconnect from the engine when it's not required (almost never) so it would make very little, if any, difference to the draw on the engine.
I could be wrong though.

It does disconnect on some cars (such as newer bmws) and just connects under braking.

glenh – Member

sharkbait – Member

Surely the alternator is always producing power? It doesn't disconnect from the engine when it's not required (almost never) so it would make very little, if any, difference to the draw on the engine.
I could be wrong though.

It does disconnect on some cars (such as newer bmws) and just connects under braking.

in fact, the volvo 480 (1989 onwards) used to disconnect the rear heated screen when you floored the throttle so as to cut loads on the engine and move you faster. Great idea imo

I think you'll find air density is the issue. As it gets colder the air density increases, this means your car has got to push harder to get through it.

I'm surprised at some of the answers on here, have none of you heard of Occam's Razor?

Is that like Shatner's Bassoon

Flash – Member
I think you'll find air density is the issue. As it gets colder the air density increases, this means your car has got to push harder to get through it.

not so sure it'd make much of a difference. According to here

http://www.engineeringtoolbox.com/air-temperature-pressure-density-d_771.html

at 0C (30F) the density is 0.081 lb/ft^3

at 20C (70F) the density is 0.075 lb/ft^3

or 8% more from mid-summer to depths of winter

The damp carpet could be a leaky heater matrix. Keep an eye on your coolant level.