Reliability of fuel consumption reported by computer

Based on how far I travel and how much fuel I put in at the pump, my
recently-acquired 1995 730i is guzzling fuel at the rate of about 17mpg.
But the computer says I am getting nearly 22mpg.

That's a huge difference. Before I go looking for a fuel leak (and I can't
smell anything) how much faith should I have in the computer's number? If
it is generally reckoned to be pretty reliable, where does it measure the
fuel flow?

Roy

In article >,
Roy Hann > wrote:
> Based on how far I travel and how much fuel I put in at the pump, my
> recently-acquired 1995 730i is guzzling fuel at the rate of about 17mpg.
> But the computer says I am getting nearly 22mpg.

When was it last re-set? If not reset it's giving an average - perhaps
over years. I reset mine each time I fill up and it then gives an accurate
figure for that tank only. And is as accurate as doing the calculation
manually.

> That's a huge difference. Before I go looking for a fuel leak (and I
> can't smell anything) how much faith should I have in the computer's
> number? If it is generally reckoned to be pretty reliable, where does
> it measure the fuel flow?

They generally count fuel injector pulses.

--
*Too many clicks spoil the browse *

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" > wrote in message
...
> In article >,
> Roy Hann > wrote:
>> Based on how far I travel and how much fuel I put in at the pump, my
>> recently-acquired 1995 730i is guzzling fuel at the rate of about 17mpg.
>> But the computer says I am getting nearly 22mpg.
>
> When was it last re-set? If not reset it's giving an average - perhaps
> over years.

Good point. I thought I had, but not yet being fully familiar with it I may
have blundered.

> I reset mine each time I fill up and it then gives an accurate
> figure for that tank only. And is as accurate as doing the calculation
> manually.

So to sum up: (1) the computer can be presumed to be reliable, and (2) it is
measuring fuel consumption as near to the point where the fuel is ignited as
possible. That's what I needed to hear.

Thanks.

Roy

In article >,
Roy Hann > wrote:
> > When was it last re-set? If not reset it's giving an average - perhaps
> > over years.

> Good point. I thought I had, but not yet being fully familiar with it I
> may have blundered.

They tend to vary slightly in how they work, but to check if it's reset
properly switch off then on again. It should read zero or blank. When it
starts displaying on the move, the figure will change often until the
miles mount up and being an average takes longer to show any change. Most
other cars have an instantaneous reading setting, but BMW give you a
separate gauge for this.

> > I reset mine each time I fill up and it then gives an accurate
> > figure for that tank only. And is as accurate as doing the calculation
> > manually.

> So to sum up: (1) the computer can be presumed to be reliable, and (2)
> it is measuring fuel consumption as near to the point where the fuel is
> ignited as possible. That's what I needed to hear.

In case it's not obvious, you can check its accuracy easily. Brim the tank
and zero it. Note the mileage, but not the fuel needed. At each subsequent
fill note fuel needed and mileage, but don't reset the computer. After a
few tanks worth brim the tank and add up fuel used against mileage.
Brimming the tank at this last time brings you back to your reference
point - before this it doesn't matter if you fill the tank or not.

--
*I was once a millionaire but my mom gave away my baseball cards

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" > wrote in message
...
> In article >,

> In case it's not obvious, you can check its accuracy easily. Brim the tank
> and zero it. Note the mileage, but not the fuel needed. At each subsequent
> fill note fuel needed and mileage, but don't reset the computer. After a
> few tanks worth brim the tank and add up fuel used against mileage.
> Brimming the tank at this last time brings you back to your reference
> point - before this it doesn't matter if you fill the tank or not.

I routinely do that with my Land Rover which doesn't run to such refinements
as a computer. :-)

Thanks.

Roy

"Roy Hann" > wrote in message
...
>> manually.
>
> So to sum up: (1) the computer can be presumed to be reliable,

Don't know about the 7 series, but on my '99 328i, the computer MPG readout
has averaged 9% high over about 45,000 miles. My 2003 Z4 has averaged 6%
high over 35,000 miles, and my 2006 R1200RT averages 3% high over 12,000
miles. While BMW seems to be improving in this area, my experience has been
that the OBC remains somewhat optimistic. Of course, as they say, YMMV!

Tom K.

"Tom K."wrote
> Don't know about the 7 series, but on my '99 328i, the computer MPG
> readout has averaged 9% high over about 45,000 miles. My 2003 Z4 has
> averaged 6% high over 35,000 miles, and my 2006 R1200RT averages 3% high
> over 12,000 miles. While BMW seems to be improving in this area, my
> experience has been that the OBC remains somewhat optimistic. Of course,
> as they say, YMMV!

According to my calculations, the comp in my '02 530i is about 3%
optimistic. The analog mpg gauge (temporary consumption) is a big fat liar
though.

Pete

My 1995 328i's computer is almost spot on when doing steady open road
travelling. As soon as I spend a lot of time in traffic, towing and
accellerating hard the computer becomes very optimistic regarding projected
fuel consumption. Sometimes more than 10% too optimistic.


"Roy Hann" > wrote in message
...
> Based on how far I travel and how much fuel I put in at the pump, my
> recently-acquired 1995 730i is guzzling fuel at the rate of about 17mpg.
> But the computer says I am getting nearly 22mpg.
>
> That's a huge difference. Before I go looking for a fuel leak (and I
> can't smell anything) how much faith should I have in the computer's
> number? If it is generally reckoned to be pretty reliable, where does it
> measure the fuel flow?
>
> Roy
>

Why do you think the instantaneous meter is a "big fat liar"? I've always
found it to be pretty reasonable at any given moment, depending how hard I'm
flogging the car.

- nopcbs
"Pete" > wrote in message
...
>
> "Tom K."wrote
>> Don't know about the 7 series, but on my '99 328i, the computer MPG
>> readout has averaged 9% high over about 45,000 miles. My 2003 Z4 has
>> averaged 6% high over 35,000 miles, and my 2006 R1200RT averages 3% high
>> over 12,000 miles. While BMW seems to be improving in this area, my
>> experience has been that the OBC remains somewhat optimistic. Of course,
>> as they say, YMMV!
>
> According to my calculations, the comp in my '02 530i is about 3%
> optimistic. The analog mpg gauge (temporary consumption) is a big fat
> liar though.
>
> Pete
>
>

While I question the Big Fat Liar statement, I can't help but wonder how you
could possibly know that it was reasonably accurate at any given moment. At
any given moment, I'm far to busy to calculate how much fuel I've consumed
over the given distance.

I like to think the consumption-of-the-moment guage is accurate, but all I
really know is that when I fill the tank and run the numbers, the calculator
and the CONSUM feature of the OBC fall in line with one an other.




"nopcbs" > wrote in message
...
> Why do you think the instantaneous meter is a "big fat liar"? I've always
> found it to be pretty reasonable at any given moment, depending how hard
> I'm flogging the car.
>
> - nopcbs
> "Pete" > wrote in message
> ...
>>
>> "Tom K."wrote
>>> Don't know about the 7 series, but on my '99 328i, the computer MPG
>>> readout has averaged 9% high over about 45,000 miles. My 2003 Z4 has
>>> averaged 6% high over 35,000 miles, and my 2006 R1200RT averages 3% high
>>> over 12,000 miles. While BMW seems to be improving in this area, my
>>> experience has been that the OBC remains somewhat optimistic. Of
>>> course, as they say, YMMV!
>>
>> According to my calculations, the comp in my '02 530i is about 3%
>> optimistic. The analog mpg gauge (temporary consumption) is a big fat
>> liar though.
>>
>> Pete
>>
>>
>
>