Menu toggle

MPG differences between summer and winter.

dirtydirtyharry

dirtydirtyharry

New member
I use my 944 for mainly motorway miles and get about 29/30 pmg during the summer. Its the first winter that I have owned her and am currently getting about 24mpg. That seems a big drop to me. Is this fairly standard?

I appreciate that I have the heater running a fair bit more but I would not have expected such a drop.
 
As far as I'm aware, yes. I plotted my MPG of my daily driver last year and it was a perfect little wave, cresting in summer and dipping in winter. Its the cold air I believe, its denser and makes the car use more fuel to compensate.
 
I agree scotty I didn't buy it for it's stunning fuel economy!!! It's like when people say to you that it's not very practical is it? Who farking cares it's a Porsche and it's mine!!!! [:)]
 
Mine has dipped considerably over the winter, especially in the recent weather, but I think my thermostat is not in the best of conditions as mine has been running very cold in this bad weather
 
Scotty said "Your driving a porsche ! who cares about fuel economy? certainly not me !"

dirtydirtyharry wasn't complaining about the fuel enconomy as such, he was asking a serious qusetion about the increase in consumption that he was experiencing in the winter.
Perhaps, Scotty, rather than post a sarcastic response you should read the original post carefully or not bother to reply at all.

 
Good question. Those figures look about right to me (I have an S2, DDH I notice you don't specify what model you have.)

In the winter the air is colder and therefore denser. This has two (opposing) effects on fuel consumption.

1. Colder, denser air will weigh more for a given volume (remember, your engine works on volume not mass), and therefore will burn a little more efficiently in your engine. This will slightly reduce your fuel consumption.

2. Colder, denser air is thicker, and you will therefore need to expend more energy pushing your car through it at a given speed. This, particularly at higher speeds (DDH - you mention mainly motorway driving) will have a MUCH bigger effect than factor 1.

Therefore, in cooler conditions, you can expect your fuel economy to drop slightly. Around 10% seems to be the accepted rule of thumb, across the board. If you are driving faster on average then you can expect this percentage to go up a little. (Air resistance goes up with - I think - the third power of speed. Double your speed and you increase the drag by a factor of eight.)


Oli.
 

ORIGINAL: zcacogp

Good question. Those figures look about right to me (I have an S2, DDH I notice you don't specify what model you have.)

In the winter the air is colder and therefore denser. This has two (opposing) effects on fuel consumption.

1. Colder, denser air will weigh more for a given volume (remember, your engine works on volume not mass), and therefore will burn a little more efficiently in your engine. This will slightly reduce your fuel consumption.

2. Colder, denser air is thicker, and you will therefore need to expend more energy pushing your car through it at a given speed. This, particularly at higher speeds (DDH - you mention mainly motorway driving) will have a MUCH bigger effect than factor 1.

Therefore, in cooler conditions, you can expect your fuel economy to drop slightly. Around 10% seems to be the accepted rule of thumb, across the board. If you are driving faster on average then you can expect this percentage to go up a little. (Air resistance goes up with - I think - the third power of speed. Double your speed and you increase the drag by a factor of eight.)


Oli.
Click to expand...

Square of speed.
 
ORIGINAL: supersport
Square of speed.
Click to expand...
Ah, helpful, thanks. I stand corrected.

(And doesn't tyre and road wear go up with the fifth power of axle loading, or something daft like that?)


Oli.
 
My thoughts are that in the winter, one is using Headlights,side and rear lights, wipers,heater fan and the starter motor has to work harder turning a cold engine and all of this increased electrical demand has to be generated by the engine, so more fuel is used compared to Summer use.

This is probably a small factor compared to other reasons for increased consumption, but as they say, 'Every Little Helps'
 
As Oli says really, but also the point that the cars stay 'on choke' a lot longer each morning/evening as they take longer to warm up and dependant on the length of your journey, that could have a significant impact.
 
Just as a bit of visual proof of concept my AFR meter during the cold spell dropped down as low as 13.5 or lower. The lower the number means the richer the engine is running. This morning now it is a little warmer when on idle and warm my AFR has returned to something approaching the 14.3 mark, which is normal running for my car. Real evidence that cars do run richer and consume more fuel in cold weather [:)]
 
Rob,

Does your AFR work on mass or volume of air? (And fuel, for that matter.)

Mass for a given volume goes up as temperature goes down. If your AFR measures volume then you would expect the reading to drop with temperature, as it requires less (as it is heavier) air for a given volume of fuel.


Oli.
 
They detect the oxygen content of the exhaust gas I think. I guess it might be telling you the ratio of fuel molecules to oxygen molecules in the combustion chamber so I guess it is mass rather than volume.
 
As Tom said mate it measures the air fuel ratio by analysing the exhaust gas (innovate gauge and Bosch 5 wire sensor). I am guessing (without tearing into the manual) like he said it is working on mass. Question??? Iis this why I have sometimes heard of Air Flow Meters being referred to as Air Mass Meters? Just a thought.
 
I guess technically air flow meters measure volume rather than mass. Probably people call them the same thing half the time. The 944 uses an air flow meter, measuring volume, a common mod is to switch to an air mass meter (or Mass Air Flow MAF). Typically due to the reduction in restriction and greater measurement range of the MAF sensor.

 
Strangely my Wife complained last night about the MPG in her BMW diseasal and I was able to give her a very succint and technical reason why that was.....[:D]
 
I did that last year on my daily driver when the thermostat stuck open.

I'm glad I'm not the only one who keeps an eye on their fuel economy, I know very few people who do....
 
You must log in or register to reply here.

Posts made and opinions expressed are those of the individual forum members

Use of the Forum is subject to the Terms and Conditions

Disclaimer

The opinions expressed on this site are not necessarily those of the Club, who shall have no liability in respect of them or the accuracy of the content. The Club assumes no responsibility for any effects arising from errors or omissions.

Porsche Club Great Britain gives no warranties, guarantees or assurances and makes no representations or recommendations regarding any goods or services advertised on this site. It is the responsibility of visitors to satisfy themselves that goods and/or services supplied by any advertiser are bona fide and in no instance can the Porsche Club Great Britain be held responsible.

When responding to advertisements please ensure that you satisfy yourself of any applicable call charges on numbers not prefixed by usual "landline" STD Codes. Information can be obtained from the operator or the white pages. Before giving out ANY information regarding cars, or any other items for sale, please satisfy yourself that any potential purchaser is bona fide.

Directors of the Board of Porsche Club GB, Club Office Staff, Register Secretaries and Regional Organisers are often requested by Club members to provide information on matters connected with their cars and other matters referred to in the Club Rules. Such information, advice and assistance provided by such persons is given in good faith and is based on the personal experience and knowledge of the individual concerned.

Neither Porsche Club GB, nor any of the aforementioned, shall be under any liability in respect of any such information, advice or assistance given to members. Members are advised to consult qualified specialists for information, advice and assistance on matters connected with their cars at all times.

Porsche
Lockton
Pirelli
Officially approved
Oracle Finance
WEX Esso
Collecting Cars
Back
Top