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Old 06-09-2010, 08:42 PM   #1
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Massive difference in MPG (CLS 320 CDi)

Just thought I'd post something of interest on here about how much difference atmospheric conditions can affect mpg on these cars.

Early morning runs give me 43-45mpg whereas late afternoon runs are giving me 38-40mpg on the same route.

Quite a difference really.
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Old 06-09-2010, 08:45 PM   #2
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Are the traffic conditions the same.

I find my diesel cars run better when the temperature is warmer.
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Old 06-09-2010, 08:46 PM   #3
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Definitely atmospherics.

IC engines much prefer dense, moist cool air like you get early mornings.
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Old 06-09-2010, 08:52 PM   #4
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Definitely atmospherics.

IC engines much prefer dense, moist cool air like you get early mornings.
For making more power yes, but for the better consumption, you want it warmer. Less time for the engine to heat up, and a lesser temperature differential between the engine and the air around it, ergo less energy wasted as heat loss.
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Old 06-09-2010, 08:55 PM   #5
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For making more power yes, but for the better consumption, you want it warmer. Less time for the engine to heat up, and a lesser temperature differential between the engine and the air around it, ergo less energy wasted as heat loss.
More power is more efficiency for the same amount of fuel.
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Old 06-09-2010, 08:59 PM   #6
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More power is more efficiency for the same amount of fuel.
I'm finding the converse temp wise.

I used to have a 220cdi E class (not had the V6 long enough to say how it was in winter).

In winter time I struggled to get 40mpg from it in winter, usually 35 on my commute, in summer this was about 42-45 and into the 50's.
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Old 06-09-2010, 09:14 PM   #7
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More power is more efficiency for the same amount of fuel.
Colder are is denser, so more wind resistance, so more power needed.

Or not. Sounds vaguely plausible
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Old 06-09-2010, 09:15 PM   #8
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Colder are is denser, so more wind resistance, so more power needed.
So more fuel used?
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Old 06-09-2010, 09:17 PM   #9
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My tests showed that my car gave best efficiency on long journeys when the ambient temperature was ~10C.
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Old 06-09-2010, 09:53 PM   #10
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So more fuel used?
Yes, to over come the wind resistance. And no, as colder air produces more power from the same amount of fuel. I suspect....

My car is the same all year round.
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Old 06-09-2010, 10:07 PM   #11
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Maybe I'm lighter footed in the mornings as I don't want to go to work and heavier footed on the way home.
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Old 06-09-2010, 10:13 PM   #12
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Maybe I'm lighter footed in the mornings as I don't want to go to work and heavier footed on the way home.
Thats my kinda logic
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Old 06-09-2010, 10:20 PM   #13
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My particular diesel car gives much better MPG in warmer temperatures, the difference is actually quite significant. Same trip in winter/summer gives 35/44 so 9mpg!
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Old 06-09-2010, 10:48 PM   #14
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I'm finding the converse temp wise.

I used to have a 220cdi E class (not had the V6 long enough to say how it was in winter).

In winter time I struggled to get 40mpg from it in winter, usually 35 on my commute, in summer this was about 42-45 and into the 50's.
When warmer - better grip.
Colder - less grip so more wheels spin (even if you dont feel it)

More work needed so you burn more fuel. You "work" harder to achive the same result on driveability. Its hard to find balance between low temperature, grip, accelaration, driving conditions, traffic, tyre pressure, weight, wind, qualityu of fuel, average speed, atmosferic pressure, etc, etc, etc... to get better or worst mpg.

Im a Merc driver for a while and I always been concern about mpg and I found when you drive a car like this you should enjoy it fully or sell it.

One time you will get 18-22mpg and other times you will get 45+.

Happy driving.
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Old 07-09-2010, 12:31 AM   #15
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I remember hearing that you should always fill your car up early in the morning when the fuel is colder and therefore more dense.

Probably get an extra litre per year on 1 fill per week.

But I know some people who will only half fill their cars so they are carrying less weight - probably wasted on the extra trips to the garage.

My MPG got remarkably better on the trip computer when I got the gearbox oil changed - now it just tells me bigger lies. It used to be 4mpg out and now it is between 6-8. It does make me fell better when I see 50mpg on a long run even though I know it is BS.
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