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Posted by Stuart on July 26, 2009, 7:02 pm


> I decided to try a few experiments on fuel economy.

> Equipment used:
> Mpg meter on Golf's dashboard

Unfortunately I have proved such devices to be inaccurate.

There is only one way of ensuring reliable mpg figures, that is to fill up
your tank, do some miles and fill your tank again - at the same pump.

Garage fuel pumps are calibrated devices subject to spot checks by HM
weights and measures inspectors. They have to be right. Using the same
pump ensures that the "full" point will not be affected by varying
sensitivities in pump shut-off devices or whether the car is standing on
level ground.

I assume you are using your TomTom to measure your speed and distance
rather than your speedometer.

You also need to ensure your tyre pressures and weather conditions (wind
speed and direction) are exactly the same and you use exactly the same
section of road for your speed test to eliminate the effects of road
gradient.

As far as "driving sensibly" is concerned, smooth, gentle acceleration,
changing up as soon as possible and keeping in the highest gear the car
can manage comfortably, is only half, literally half, the story.
Deceleration needs to be similarly controlled, anticipating hazards and
trying to slow gently without using your brakes unless necessary. Proper
observation and anticipation at islands /can/ often allow you to slip into
a gap in the traffic coming round without braking at all. Similarly, try
to time your arrival at traffic lights to co-incide with a green phase.

As far as "holding up other traffic" is concerned, you should be aiming to
keep to the speed limit at all times - up to it as well as down to it -
that does mean doing 70 not 60 where it is appropriate.


Posted by Rod Speed on July 27, 2009, 4:09 pm


Stuart wrote

>> I decided to try a few experiments on fuel economy.

>> Equipment used:

>> Mpg meter on Golf's dashboard

> Unfortunately I have proved such devices to be inaccurate.

> There is only one way of ensuring reliable mpg figures, that is to fill up
> your tank, do some miles and fill your tank again - at the same pump.

Even that isnt enough.

> Garage fuel pumps are calibrated devices subject to spot checks
> by HM weights and measures inspectors. They have to be right.

They cant ever be. The temperature of the fuel makes a big difference.

Essentially because they actually measure volume, not weight.

> Using the same pump ensures that the "full" point will not
> be affected by varying sensitivities in pump shut-off devices

Wrong, because they arent that reproducible.

> or whether the car is standing on level ground.

Pity about the temperature of the fuel.

> I assume you are using your TomTom to measure your
> speed and distance rather than your speedometer.

> You also need to ensure your tyre pressures and weather conditions
> (wind speed and direction) are exactly the same and you use exactly
> the same section of road for your speed test to eliminate the effects
> of road gradient.

> As far as "driving sensibly" is concerned, smooth, gentle
> acceleration, changing up as soon as possible and keeping in the
> highest gear the car can manage comfortably, is only half, literally
> half, the story. Deceleration needs to be similarly controlled,
> anticipating hazards and trying to slow gently without using your
> brakes unless necessary. Proper observation and anticipation at
> islands /can/ often allow you to slip into a gap in the traffic
> coming round without braking at all. Similarly, try to time your
> arrival at traffic lights to co-incide with a green phase.

> As far as "holding up other traffic" is concerned, you should be
> aiming to keep to the speed limit at all times - up to it as well as
> down to it - that does mean doing 70 not 60 where it is appropriate.



Posted by <me9 on July 28, 2009, 7:30 am


On 27 Jul,

>
> They cant ever be. The temperature of the fuel makes a big difference.
>
> Essentially because they actually measure volume, not weight.
>
The fuel is nearly always stored underground, where temperature differences
are small. In comparing MPG it will be negligible in the scheme of things.


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Posted by Stuart on July 28, 2009, 9:59 am


> On 27 Jul,

> >
> > They cant ever be. The temperature of the fuel makes a big difference.
> >
> > Essentially because they actually measure volume, not weight.
> >
> The fuel is nearly always stored underground, where temperature
> differences are small. In comparing MPG it will be negligible in the
> scheme of things.

Precisely what I was going to say, I have been digging to find the
following:

The coefficient of expansion for diesel is 0.00050/degF


Posted by Rod Speed on July 28, 2009, 2:17 pm


Stuart wrote

>>> They cant ever be. The temperature of the fuel makes a big difference.

>>> Essentially because they actually measure volume, not weight.

>> The fuel is nearly always stored underground, where temperature
>> differences are small. In comparing MPG it will be negligible in the
>> scheme of things.

> Precisely what I was going to say,

Pity that its moved around in above ground tankers.

> I have been digging to find the following:

> The coefficient of expansion for diesel is 0.00050/degF

Its the main reason for the variation in the accuracy of fuel pump meters, after
the even bigger variable of not being able to fill it to the same level
accurately.



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