Another MPG thread!

[littleowl]

I've got a C220 AMG Sport Edition (lowered suspension). I've had it from April, and MPG-wise has been pretty good - best was over 64mpg on a 165 mile motorway trip down to see family. I do this trip regularly, but a month or so ago only managed 55mpg, and have noticed since then when I do a regular trip to town (approx 10 miles) I am only managing about 45mpg whereas before it would be 50.

I understand that wet roads/temperature/utilising aircon/heated seats/tyre pressures etc will have a bearing on the fuel consumption but even on days that are dry and I'm not using any extras, it seems to be considerably down on the mpg.

As I use supermarket fuel (sharp intake of breath!), can it be that the change to winter fuel can make that much difference on mpg? Next fill up I'm going to try premium diesel and see what happens.

 

[whitenemesis]

Ambient temperatures are significantly lower now and so has a large impact on mpg particularly on diesels.
I don't think fuel suppliers have switched to winter formula. Not even sure they do in the UK?

 

[markjay]

And we should keep in ming that the 'gallon' in 'miles per gallon' is a unit of volume, not mass.

As such, the caloric value of a gallon of fuel will be higher when the fuel is cold (because you get more fuel mass per gallon).

So your car will go further on a gallon of fuel at 0 degrees than it would on a gallon of fuel at 30 degrees.

(And if you are a real money fincher... always refuel at 4am, you'll get more fuel for your money).

 

[190]

Winter fuel will account for 2 or 3% and you can subtract a similar amount for increased aerodynamic drag due to lower temperatures. Without doing the sums again, aero drag is actually a bigger effect than winter fuel at motorway speeds. Tyre pressure effects are variable depending speed and smaller than many people think. Typically 10% lower pressure will only increase mpg by 0.8% Added all together they would struggle to account for your difference at least on the long trips when extended warm up times are not so much a factor.


How about wind speed and direction on the motor way trips ?

 

[72 dudes]

I don't think fuel suppliers have switched to winter formula. Not even sure they do in the UK?

Yes, they do. I work p/t hours at a Sainsburys petrol station. IIRC, winter diesel starts late October and runs through to early March.

The OP's MPG sounds good to me, even if it has dropped recently. I assume he's confident it's not a dirty air filter or binding brakes etc?

 

[GeeJayW]

I've got a C220 AMG Sport Edition (lowered suspension). I've had it from April, and MPG-wise has been pretty good - best was over 64mpg on a 165 mile motorway trip down to see family. I do this trip regularly, but a month or so ago only managed 55mpg, and have noticed since then when I do a regular trip to town (approx 10 miles) I am only managing about 45mpg whereas before it would be 50.

I understand that wet roads/temperature/utilising aircon/heated seats/tyre pressures etc will have a bearing on the fuel consumption but even on days that are dry and I'm not using any extras, it seems to be considerably down on the mpg.

As I use supermarket fuel (sharp intake of breath!), can it be that the change to winter fuel can make that much difference on mpg? Next fill up I'm going to try premium diesel and see what happens.

The fuel formulation may have changed for example to include more biofuel components (which contain oxygen which isn't a fuel) or maybe even a different base fuel with lower density (but still within EN:590). It's also worth checking your tyre pressures, they do drop a little when it's cold.

 

[190]

I think the dates for seasonal fuel specification are cast in stone and it's a legal requirement for them to comply.

 

[GeeJayW]

I think the dates for seasonal fuel specification are cast in stone and it's a legal requirement for them to comply.

Just about to change apparently..
Winter diesel fuel - Wikipedia

 

[littleowl]

So your car will go further on a gallon of fuel at 0 degrees than it would on a gallon of fuel at 30 degrees.

Not 100% (or even 50% ) understanding what you're saying - does that mean I should be getting better mpg in colder temperatures?

 

[littleowl]

The OP's MPG sounds good to me, even if it has dropped recently. I assume he's confident it's not a dirty air filter or binding brakes etc?

It's not binding brakes as my friend checked, but it is due a service so possibly an air filter might be needed. I do check tyre pressures regularly and although they seem to lose air more quickly than my other cars have for some reason, I make sure they are at the right levels. BTW I'm a she

 

[190]

Not 100% (or even 50% ) understanding what you're saying - does that mean I should be getting better mpg in colder temperatures?

I'm not sure the temperature of fuel stored underground would vary all that much but if it was colder then all other things being equal you get a bit more out of denser cold fuel. All other things are not equal though and any beneficial effects of cold fuel would be swamped by the other cold temperature factors of lower calorific value and denser air.

 

[markjay]

If you measure fuel consumption by gallons needed between brim-to-brim... with the car level, using the same pump etc... then you will get more fuel mass (and more caloric value) into your tank if you feel up at 0 degrees than if you feel up at 30 degrees.

This is because the fuel expands when warm, so you will get less mass per gallon. The fuel will be more condense when cold.

In other words, your car will go further on a gallon on fuel purchased at 0 degrees than it would on a gallon of fuel purchased at 30 degrees. Hence more 'miles per gallon'.

And while the underground tank does not have the same temperature as the air above ground, it will still be colder in winter than it is in summer.

 

[markjay]

From what I found online.... a 25 gallon tank filled at -7 degrees, then warmed to 38 degrees, will increase by just over one gallon for petrol, and just under 1 gallon for diesel.

This is because the thermal expansion coefficient for Diesel fuel is .00083, while it is .00095 for petrol.

But that's over a 45 degrees difference... a 15 degrees difference (which is a more realistic figure for underground storage temperature fluctuations between summer and winter) will see the fuel in 25 gallon tank expand by around one third of gallon.

 

[Petrol Pete]

Do I remember F1 teams being banned from chilling their fuel at some point ? ...or did I just make that up ?

 

[whitenemesis]

Holy crap! We've come to this?
I just fill up with V-Power and be done with it....

 

[Mactech]

Do I remember F1 teams being banned from chilling their fuel at some point ? ...or did I just make that up ?

No you didn't make it up! 'cos I bought McLaren's refrigeration kit after it was banned in F1 and used it to win two World Championships in Sports cars in the late 80's. ......Where it was still legal!
As a rule of thumb, we saw a 1% increase in fuel density for every 10 deg C we lowered the fuel temp.
So if we put it in at -20 deg instead of +20 deg we got an effective extra 4% of fuel.
Very useful for completing the last 4 laps of a 104 lap race on a restricted volume (not mass!) of fuel.
There were some logistical issues involved, but nothing compared to the joy of winning championships.....against Mercedes and Porsche

 

[190]

I think we are all agreed that colder fuel will be more dense and therefore contain more energy.

The realistic question though is how much does the temperature of fuel change in an underground storage tank. We are talking about a huge volume in what is effectively an insulated tank. The turn over of fuel in a busy station would mean the temperature change would be very small so effectively it would be close to the temperature when delivered. To that extent it must be colder in winter than in summer but there would be no meaningful variation on a daily basis or even over several days.

My earlier point was that seasonal calorific value changes plus increased aerodynamic drag would swamp the effect of colder fuel which would need a 50 deg C temperature change to be equivalent of those two effects if they amounted to 5%.

When it comes to be dispensed, modern fuel pumps have temperature compensation built in although it's not mandatory. So in practice there will be no advantage in buying colder fuel unless you seek out pumps that are not temperature compensated. How do you tell the difference ? I don't know but will be looking closely at the pump next time I fuel up.

 

[markjay]

I think we are all agreed that colder fuel will be more dense and therefore contain more energy.

The realistic question though is how much does the temperature of fuel change in an underground storage tank. We are talking about a huge volume in what is effectively an insulated tank. The turn over of fuel in a busy station would mean the temperature change would be very small so effectively it would be close to the temperature when delivered. To that extent it must be colder in winter than in summer but there would be no meaningful variation on a daily basis or even over several days.

My earlier point was that seasonal calorific value changes plus increased aerodynamic drag would swamp the effect of colder fuel which would need a 50 deg C temperature change to be equivalent of those two effects if they amounted to 5%.

When it comes to be dispensed, modern fuel pumps have temperature compensation built in although it's not mandatory. So in practice there will be no advantage in buying colder fuel unless you seek out pumps that are not temperature compensated. How do you tell the difference ? I don't know but will be looking closely at the pump next time I fuel up.

There are a couple of other points to consider.

Underground fuel tank are insulted. Fuel delivery lorries, are not.

In a petrol station with quick turnover, the fuel in the underground tanks will be roughly at the same temperature as it was hen delivered from the tanker.

And, the fuel in the pump and the hose will be closer to ambient temperature - so if you only fill-up a few litres, most of it will be at ambient temperature.

In short - yes, colder fuel will provide more miles per gallon - but it is very difficult aiming at filling-up with cold fuel.

It is probably fair to say that on average the fuel will be colder during winter months than in summer months. But this does not mean that in winter every tankful will always be colder fuel.

 

[popuptoaster]

I think if you are worrying about chilling fuel to get more MPG, buying a big MB was probably not the right choice

Mine does about 16-17mpg average but it's all A and b roads for me, the few times I've done a motorway run it gets low to mid 20's provided i don't get excited.

 

[Petrol Pete]

OK so lets get a bit (more) silly, the fuel tank in most cars hangs pretty much exposed under the car. So in a freezing cold winter make sure you fill up everyday before you arrive home every day from work (yes ,yes I realise most MB owners are millionaires who keep their cars in temperature controlled garages , but bear with me.) That way you will have ice cold fuel every morning ! simples !, My advice is not based on any type of science , real or imagined.
Disclaimer: all of the above does not count during the summer months...or if you live in Dubai.

ill get my coat.