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Discussion Starter · #1 ·
Hello,
I was curious ... Does anybody find the gas mileage indicator of any use ?
One thing I've noticed is that whenever I brake to come to a complete
stop, it swings to the right really quick before settling to the far left.
Is this some kind of "average" indication or am I dreaming ?

Thanks,
 

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demosa said:
Hello,
I was curious ... Does anybody find the gas mileage indicator of any use ?
One thing I've noticed is that whenever I brake to come to a complete
stop, it swings to the right really quick before settling to the far left.
Is this some kind of "average" indication or am I dreaming ?

Thanks,
The indicator is an instantaneous indication of fuel consumption. It swings to the right indicating the (high) fuel consumption at that instant. It does swing back to the left at idle, which in theory would be the highest possible fuel consumption.

Is it useful? It is a constant reminder of your driving habits. If it swings to the right you are reminded that you may be wasting fuel. That is a good thing but it does suck the fun out of more spirited driving.

I find the long-term fuel consumption computer useful.

-Neil
 

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Discussion Starter · #3 ·
Thanks for the reply,
Yes, I know how it works but I'm pointing out some strange behavior
when I come to a complete stop. Say I'm going at 10 miles per
hour, I'm in netrual and I'm riding the brakes smoothly till I come
to a complete stop. Right before the stop, the indicator swings to
the right and then settles to the left. I'm not doing anything with
the gas pedal.....
:dunno:
 

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demosa said:
Thanks for the reply,
Yes, I know how it works but I'm pointing out some strange behavior
when I come to a complete stop. Say I'm going at 10 miles per
hour, I'm in netrual and I'm riding the brakes smoothly till I come
to a complete stop. Right before the stop, the indicator swings to
the right and then settles to the left. I'm not doing anything with
the gas pedal.....
:dunno:
I might be mistaken, but isn't the MPG gauge just some type of vacuum gauge?
 

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demosa said:
Thanks for the reply,
Yes, I know how it works but I'm pointing out some strange behavior
when I come to a complete stop. Say I'm going at 10 miles per
hour, I'm in netrual and I'm riding the brakes smoothly till I come
to a complete stop. Right before the stop, the indicator swings to
the right and then settles to the left. I'm not doing anything with
the gas pedal.....
:dunno:
It's the meter's way of telling you that, as you approach complete stop, your instantaneous mileage goes to almost nothing (swing to the right). Once you've stopped, the meter swings all the way to the left (call it the "off" or "N/A" position) to tell you that, because you're sitting still, mileage calculation is pointless and therefore "off". There's no point in calculating gas mileage, even if the engine is burning fuel at idle, unless the wheels are moving you forward; you're getting "negative" gas mileage at that point.
 

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eston said:
I might be mistaken, but isn't the MPG gauge just some type of vacuum gauge?
I have been curious about how this works as well.

Ideally, you would divide the vehicle's current speed by the current gas/throttle level to get an instantaneous MPG. If it's just a vacuum gauge I'm not sure how it could do a "calculation" like this.

I do find it an useful tool to help learn and reinforce which driving habits consume the most fuel.
 

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demosa said:
Hello,
I was curious ... Does anybody find the gas mileage indicator of any use ?
No. I find it completely irrelavent and annoying. I am completely mystified why engineers building the "Ultimate Driving Machine" found it necessary to provide this feature.

Personally I would have rather had an oil temperature gauge. Actually I was surprised during the test drive to find out it was NOT the oil temperature gauge... :dunno:
 

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My TARDIS is a BMW
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wingspan said:
No. I find it completely irrelavent and annoying. I am completely mystified why engineers building the "Ultimate Driving Machine" found it necessary to provide this feature.

Personally I would have rather had an oil temperature gauge. Actually I was surprised during the test drive to find out it was NOT the oil temperature gauge... :dunno:
:stupid:

I'd rather have a more useful gauge, such as oil temp, than the mileage indicator. But sometimes the "fun meter" is kind of amusing.
 

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Rob325_in_AZ said:
I have been curious about how this works as well.

Ideally, you would divide the vehicle's current speed by the current gas/throttle level to get an instantaneous MPG. If it's just a vacuum gauge I'm not sure how it could do a "calculation" like this.

I do find it an useful tool to help learn and reinforce which driving habits consume the most fuel.
It measures manifold presure (or how hard your engine "sucks" exactly). Assuming that you're mixture is not too rich or too lean, that would be relatively good approximation on how much fuel you're using.
 

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Ik hou van Holland...
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They stuck these meters into BMW's because in Europe the gas is so expensive and want to make the driver aware of the usage...:p For me I think that Oil temp gauge would be the better way to go, as said before...:thumbup:
 

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It swings to the right first, even when coasting to a stop in neutral, because at idle you are consuming fuel at the same rate and as you slow down you are going fewer and fewer miles at that consumtion rate. Once you stop it would be a divide by zero error (which in computers is REALLY bad) so it just goes to it's off position.

I decided to expand on this since I am so bored. So here is an example. Lets say at idle the car burns 1 gallon of gas in 1 hour. It probably doesn't but it'll make the example easier.

At 35 Mph you decide to come to a stop so you put the car in neutral. Right now in neutral it's getting 35 miles per gallon (35Mph at 1Gallon an hour burn rate)

Now put on the brakes and slow to 20Mph. At idle the computer thinks it can go 20 miles at 1 gallon/hour or 20 miles per gallon so the needle starts to swing right.

At 5Mph the idling engine is still going through 1 gallon of gas an hour but the cars computer thinks it will only go 5 miles in this hour or 5 MPG.

At 0 miles per hour it errors out and shuts off.

James. Yes, I need a life :eek:
 

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demosa said:
Hello,
I was curious ... Does anybody find the gas mileage indicator of any use ?
One thing I've noticed is that whenever I brake to come to a complete
stop, it swings to the right really quick before settling to the far left.
Is this some kind of "average" indication or am I dreaming ?

Thanks,
They probably put it in to appease the greens in Germany more than anything else. I actually do have use for it: on very rare days when I am after mpg instead of rpms :p, I'd set it on cruise and monitor the gauge. If I am going downhill and the gauge hits ~ 45 mpg, I shift it in neutral and let it coast. It maintains the exact same speed as before with minimal fuel consumption. That's how I get 35+ mpg on a long trip ;)
 

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brave1heart said:
They probably put it in to appease the greens in Germany more than anything else. I actually do have use for it: on very rare days when I am after mpg instead of rpms :p, I'd set it on cruise and monitor the gauge. If I am going downhill and the gauge hits ~ 45 mpg, I shift it in neutral and let it coast. It maintains the exact same speed as before with minimal fuel consumption. That's how I get 35+ mpg on a long trip ;)
Not to change the subject here...but sometimes shifting in to neutral while going down hill costs more fuel since the engine has to idle instead of free wheel in 5th..which I think turns the injectors off..?

I am sure you already know this but I am bored and felt like posting something :p.

James.
 

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James said:
Not to change the subject here...but sometimes shifting in to neutral while going down hill costs more fuel since the engine has to idle instead of free wheel in 5th..which I think turns the injectors off..?
Yes. Plus, it's not a great idea to coast in neutral cause you have disconnected acceleration ability and engine braking, both of which help you maintain control of the car.
 

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I believe they used to be vacuum based, but these days I'm sure it's all electronic. It's trivial for the DME to take instantaneous speed and fuel consumption rate and spit out a value for the gauge. The behavior with the gauge moving right then left when coming to a stop shows that it looks at at least 2 parameters.

FWIW, my M6 deletes the MPG gauge and replaces it with... a ///M badge (the only one inside the car)
 
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