# More about MPG



## Rapide561

Hi

The last tankful has yielded just over 21 mpg compared to the long term average of 24.42, and a best of over 26 - the second most recent tankful. 

Could the really cold weather cause a drop? I was on free flowing motorways, usual speed and no wind what so ever. Seems strange to be 20% down on last journey. Using Tesco fuel as always. 

Russell


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## Pudsey_Bear

The cold weather should actually make anything run better as there air is much more dense, however the extra use of heaters, and wipers will most likely negate any savings.

Kev.


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## framptoncottrell

I don't use the motorhome much in winter so I cannot make a direct comparison, but my Honda car goes from 41mpg in the summer months to 35mpg in the winter. It's just the length of time that the engine oil takes to warm up and the extra drag this causes. My impression - very unscientific, I know - is that winter traffic also tends to be more stop-and-start.

Dr (musical, not medical) Roy


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## hblewett

I agree with Dr Roy - I have always found this with my cars. In addition, as far as diesel is concerned, it has additive - up to 3% I believe - to stop it freezing in low temperatures, so your getting less actual diesel per gallon than you get in the summer for each gallon you buy.


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## DTPCHEMICALS

our scudo returns 33-34mpg. Checked on a fill to fill basis not the computer.
Usualy fill up every 10 days. Son in law drives out and i drive return journey.
Weather and load do not seem to make much difference.

Dave p


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## Boff

Hi!



Kev_n_Liz said:


> The cold weather should actually make anything run better as there air is much more dense ...


Not anything.

Firstly, the increase of air density when going from +20 to -10 degrees Centigrade is only about 10%, not more. Secondly, the only thing that might run better is the engine's internal combustion cycle itself. Everything else will run worse:

The increase in air density will also lead to an increase in air drag, which will probably already overcompensate the increase in thermodynamic engine efficiency.

As framptoncottrell has already mentioned, oil and grease will have a higher viscosity at low temperatures, leading to an increase in friction. Admittedly this is - thanks to modern multi-grade engine oils - not so much of a problem in the engine itself. But think of all the other moving, lubricated parts, like gearbox, drive shaft, wheel bearings etc.

Days are shorter in winter, and periods of low visibility due to precipitation, fog etc. are more common. Which leads to increased use of lights, especially head and fog lights. Therefore the alternator demands more energy from the engine, thus less MPG.

Starting the colder engine demands more energy from the battery, which has later to be replenished by the alternator.

Last but not least, the increased use of electric heaters (rear window, mirrors, seats etc.) and heating fans will also have an impact on MPG.

Best Regards,
Gerhard


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## Pudsey_Bear

I'm not one to argue, :lol: :lol: :lol:, but if all this is true, then the motor/van/truck manufacturers have waisted millions designing and fitting intercoolers to almost all of the quality diesel engines, who's sole purpose is to cool the incoming air to make it more dense before it goes into the turbo, anyone who has had a diesel turbo but no intercooler will say it lacks the power of intercooled versions.

As for the effects of drag etc, point taken.

Kev.


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## Hezbez

My Mondeo 2.0 TDCI returns an average of 52mpg in the summer.
In winter this drops to around 44mpg. I put it down to running with the electric heated seats, front and rear heated windscreen, lights on etc, on most of the time.


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## Rainbow-Chasers

Your diesel gets thicker - especially cheap supermarket fuel which lacks much of the additives that the large fuel companies have. Just uses more energy to pull it through, thus less to the gallon.

(more technical than that - it's just the simple version!)


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## Boff

Hi!



Kev_n_Liz said:


> I'm not one to argue, :lol: :lol: :lol:, but if all this is true, then the motor/van/truck manufacturers have waisted millions designing and fitting intercoolers to almost all of the quality diesel engines ...


Sorry, this was my insufficient command of the English language.  I should have said: Not everything!

It is certainly true that an increase in (intake) air density leads to an increase in engine efficiency. As I have said. So the money for developing intercoolers for turbocharged engines has not at all been wasted.

However: Firstly, I would assume that a modern intercooler achieves a significantly steeper drop in air temperature than 30 degrees C. And secondly, the intercooler only affects the intake air, not the air flow around the vehicle. So with an intercooler you benefit from increased air density inside the engine, while leaving the air density outside of the vehicle, and thus the air drag, unchanged.

And when increasing _ambient_ air density, the increase in air drag will be directly proportional to the density increase, while the increase in engine efficiency will be less than that.

Best Regards,
Gerhard


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## RichardnGill

Our M/H normally does 21 + mpg no problem. But we have just got back from a 200 miles trip and the best I could get was 18.5 MPG.

That is with Tesco fuel all year around.

I have a Passat 2.0 TDI and this is down to 45 MPG from averaging well over 50 MPG normally


Richard...


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## Pudsey_Bear

Your English is fine Gerhard.

I think we agree, As I said originally, the engine will perform better, but other forces conspire to make it use more fuel to run all the ancillaries, heater wipers etc. 

I remember well when I used to ride my Yamaha XS 650, it was always more fun to ride at night because of the greater density from the cooler air.

Kev.


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## Boff

Hi!



Kev_n_Liz said:


> I think we agree, As I said originally, the engine will perform better, but other forces conspire to make it use more fuel to run all the ancillaries, heater wipers etc.


Yes, we agree that the engine - as soon as it has warmed up - will perform better.

However, I have meanwhile done the maths and found that with increasing ambient air density the increase in air drag will - at least within typical European temperature ranges - always overcompensate the increase in engine performance. So there will always be less MPG at lower temperatures, even if we ignore the ancillaries.

Best Regards,
Gerhard


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## Rapide561

*MPG*

Thank you for all the replies.

I am on a 200 mile trip on Thursday so will update the thread. I did about 50 miles after refuelling and the computer shows 24.2 mpg, so in reality about 23 mpg at the computer seems "generous"

Russell


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## Jezport

Anyone who is so worried about MPG............................
Why are you driving a big brick on wheels that costs thousands to buy. I think you have chosen the wrong hobby/mode of transport/means of vacation etc.


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## peedee

So you also have a fuel guzzling 6 lire V8 car Jezport?

Given the choice between two similar layout and sized mortorhomes , one does 8mpg the other 20mpg which one would you buy?

Thanks for an interesting thread guys

peedee


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## Codfinger

*mpg*

Just to add my five bobs worth, on our xmas trip to Sufolk I did notice that we used more fuel than usual for the trip,not sure on the ratio but it was noticable.


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## Rapide561

*MPG*



Jezport said:


> Anyone who is so worried about MPG............................
> Why are you driving a big brick on wheels that costs thousands to buy. I think you have chosen the wrong hobby/mode of transport/means of vacation etc.


Hi

Not worried re the mpg, but wondered why there could be a 20% variation, driven at similar speeds, similar weather conditions etc

Russell


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## wakk44

I have noticed a marked decrease in fuel economy during the recent cold weather.Our Nissan Qashqai which has been doing 48 mpg has gone down to 40 mpg


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## Jezport

peedee said:


> So you also have a fuel guzzling 6 lire V8 car Jezport?
> 
> Given the choice between two similar layout and sized mortorhomes , one does 8mpg the other 20mpg which one would you buy?
> 
> peedee


My car does around 17 MPG around town I could drive the same sized car and get 45mpg so youve asked the question to the wrong person.


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## RichardnGill

MPG is a common question 

even if you have more money than sense and dont care how much fuel you use surely you should care about the environment.

For what its worth over my annual mileage of 8,000 miles in the M/H the cost saving of a few MPG is easily enough to pay for a few nights campsite fees. I dont drive slow (normally 65 = MPH) but do try for economy.


Richard...


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## peedee

Ah so you do have a 6litre V8 gas guzzler then  

peedee


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## Jezport

peedee said:


> Ah so you do have a 6litre V8 gas guzzler then
> 
> peedee


Not a bad guess, 2.5ltr Modified(not for economy) V6 with 4 speed auto transmission with modified torque converter.


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