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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.

They generally count fuel injector pulses.
--
*Too many clicks spoil the browse *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.

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 snipped-for-privacy@davenoise.co.uk London SW
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I routinely do that with my Land Rover which doesn't run to such refinements as a computer. :-)
Thanks.
Roy
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"Tom K."wrote

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The ROVER 827 I had (Sterling to you guys in the US _ Mk 1 version in UK) had an onboard computer system that gave instant readings instantly updated (also an average total on another button) It was interesting to watch the figures go up/down as you lifted the right foot - better than TV!

Bognor Regis, W.Sussex, England, UK
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

It's common with most OBC apart from BMW who stuck with their gauge for the instantaneous reading. Dunno if they still do on the latest cars.
--
*I believe five out of four people have trouble with fractions. *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload


My 1994 BMW 3 Series gives my instant consumption data as well. My question was, how does one determine the data is accurate at any given time? (I wouldn't ask the question, but somebody reported that it was reasonably accurate at any given moment.)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"nopcbs" wrote

Because I'm able to compare it with the digital readout of temporary consumption on the dash (it's one of those hidden features that you can display in the dashboard cluster). So, either the digital display is lying or the analog one is lying. The analog gauge would show close to 40 mpg when leasurely hwy cruising, yet, after travelling some distance like that, the OBC CONSUM (which is also optimistic, but only by about 3% in my case, as I wrote earlier) I struggle to get better than 27 mpg.
Pete
I've always

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

My longest regular trip in the UK is from London to Aberdeen - roughly 550 miles and mainly motorway. My 528 auto does 32 mpg on this journey - and I'm not taking it easy. I can't see a 530 being so much worse especially if trying for economy. The best I've seen is 38mpg over some 200 miles when acting as tender to an old Land Rover which could only cruise at 50 - and that was over mixed roads.
When I first got the 528 I checked the OBC against my own calculations over several tanks of fuel and found it accurate to better than 1%. Of course this doesn't take into account the accuracy of the mileometer, but road tests say it's ok.
--
*Forget about World Peace...Visualize using your turn signal.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"Dave Plowman (News)" wrote

32 mpg UK, right? If so, that's about 26-27 mpg US, so we're in the same ballpark then. I think I've gotten as high as 30 mpg, but that was me going at around 55-60 mph. Normally I cruise at around 70 mph.
Pete
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.