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nobbyken
Joined: 07 Jun 2006
Location: Yongin ^^
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 Posted: Sun Jul 01, 2007 6:53 pm Post subject: Fuel economy query? |
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Hi,
I drive a Matiz (800cc) and I used to drive an 800cc in the UK.
Both cars deliver better distances per litre of fuel when the fuel tank is
kept between half-full and full.
When the tank is in the bottom half of the tank, the mileage/Km rate per litre drops off noticeably.
What I would like to know is: why should cars be more fuel efficient with a full tank?
As I said, this is my second such motor and have found this to be true in both. What I don't understand is how.
I know there are many intelligent people who peruse this forum, and maybe one could enlighten.
Thanks,
Ken |
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chevro1et
Joined: 01 Feb 2007
Location: Busan, ROK
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| Posted: Sun Jul 01, 2007 7:29 pm Post subject: |
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| Sounds to me like there is some sort of inaccuracy in your fuel gauge... the 'top half' of what the gauge shows is likely more than 50% of the capacity of the fuel tank, the 'bottom half' according to the gauge is less than 50% of the actual. |
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nobbyken
Joined: 07 Jun 2006
Location: Yongin ^^
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| Posted: Sun Jul 01, 2007 8:06 pm Post subject: |
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You are right chevro1et,
For the sake of curiosity I keep a note of the litres taken to refuel to full and then find out what mileage I got to the litre (I zero the trip meter at every fill-up).
The difference can be between 11 & 16 km per litre.
I know things like driving, trip style and AC use make a difference.
In my last car (in the UK), the English manual advised to keep the car between half and full for optimum engine performance.
I never could figure why.
Running the tank to near empty, causes less mileage per litre?
Any body ever give it a try? |
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mrsquirrel
Joined: 13 Dec 2006
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| Posted: Sun Jul 01, 2007 8:30 pm Post subject: |
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| My Volvo used to be full until all of a sudden it nearly empty. Volvo's fuel gauges were notorious for it. |
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eamo
Joined: 08 Mar 2003
Location: Shepherd's Bush, 1964.
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| Posted: Sun Jul 01, 2007 9:43 pm Post subject: |
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| Common sense would lead one to believe that a lighter car gets better fuel economy. I don't understand how having a heavier fuel tank can save fuel. |
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nobbyken
Joined: 07 Jun 2006
Location: Yongin ^^
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| Posted: Sun Jul 01, 2007 10:39 pm Post subject: |
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| I would like Adam & Jamie to investigate this, I can't figure it out. |
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jay-shi
Joined: 09 May 2004
Location: On tour
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| Posted: Thu Jul 05, 2007 5:30 am Post subject: |
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| nobbyken wrote: |
my last car (in the UK), the English manual advised to keep the car between half and full for optimum engine performance.
I never could figure why. |
I can't remember for the life of me where I heard this but I think it has to do with powder or other metal residue in the gasoline in your tank. Think of it as Kool-Aid. If the tank is fuller then the residue gets passed through the combustion precess over a longer period than it would driving on an emptier tank.
It kind of makes sense. But then again I am not an expert. |
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nobbyken
Joined: 07 Jun 2006
Location: Yongin ^^
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| Posted: Thu Jul 05, 2007 5:50 am Post subject: |
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I searched loads of sites the other day (bored at work!), and I found one which said that more fuel evaporates in a tank of mostly air.
Fuller tanks have less air in them, so less evaporation.
The time factor of the frequency of trips also comes into account.
Probably not exciting enough of a topic for a Mythbusters special, but not knowing these things ways keeps you looking for the answer. 
Thanks,
Ken |
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dogshed
Joined: 28 Apr 2006
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| Posted: Thu Jul 05, 2007 2:06 pm Post subject: |
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| jay-shi wrote: |
| nobbyken wrote: |
my last car (in the UK), the English manual advised to keep the car between half and full for optimum engine performance.
I never could figure why. |
I can't remember for the life of me where I heard this but I think it has to do with powder or other metal residue in the gasoline in your tank. Think of it as Kool-Aid. If the tank is fuller then the residue gets passed through the combustion precess over a longer period than it would driving on an emptier tank.
It kind of makes sense. But then again I am not an expert. |
Keeping the tank full reduces condensation and the tank will then gain less water. Maybe that's what you heard. A small amount of water is miscible in the gasoline and that's how water is removed from the fuel tank. How much water is removed would depend on the total amount used and not how full the tank is.
However, if you keep the tank full then there will be less water to begin with due to the condensation reduction.
The water that does not mix with the gasoline, which is most of it, settles to the bottom below the opening that takes fuel to the engine. Hopefully, the water stays below this line. |
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