That's why I said it wouldn't be perfect, but more than 6 litres is still an awful lot to be out by. It'd probably fill up the tank filler pipe several times over!
When CB radio first came out over here, people discovered that some of the illegal extra high powered transmitters would scramble the brains of electronic pumps. They'd complain at the till if the resulting silly reading was high...
Colin.
When I got my 1995 NG900 I was told the SID mpg reading was not accurate. Not believing what I was told I decided to use it accurately. I always fill to the brim record the figures and reset the SID, I accept that pump cut off points can be slightly different. (If I owned a petrol station I would set the cut off points high so that I sold more!). Most of the time I use the same station and the same pump. I record the actual mpg and the SID mpg at each fill up and then reset the trip and the SID. Over ten thousand miles the actual and SID figures are very similar and usually within one mpg with the SID figure slightly lower than the actual. I am very impressed with the computers accuracy. Incidentally other peoples mpg figures are not usually worth considering as they are rarely arrived at accurately, such as I did x miles on half a tank!
None but I did mention the accuracy of replicating the driving a little earlier. :)
2 mpg over what distance? This is the precise point that's being made. Can you say that you didn't be just a little lighter on the loud pedal for part or all of the journey? The answer has to be no because unless the test is over precisely controlled conditions, you can't make such claims other than implied.
I used to do a journey where I could do anything from 26 mpg to 34.2 mpg and all I did was drive more sedately. Where would those figures be if I'd changed the spark plugs and driven somewhere in the middle of both driving styles? Then add in the other error factors and we're back to square one.
David.
Do Saab provide a calibration certificate for the computer or the components in its constructions?
I don't have one! :)
What's the calibration and component error there?
David.
You're quite right that any claim of improved mpg is difficult to establish. The various controversial 'economy' devices is a very good example of how people can be hoodwinked. We only have the primitive tools we've got. Nevertheless, my point was that relative measurements are probably more reliable that absolute measurements since at least the bias is taken out.
I drive approximately 35 miles to work, mostly on motorway without stopping, so I can control the speed. I observe not only average consumption, but also the range of consumptions between various days. I know the road gradients in great details and know where I can expect more and when I can expect less consumption. I also know that the consumption can quickly be destroyed by heavy use of the pedal, so I consciously try the obtain the best figures for a particular day; competing with myself if you like. It is not scientific by any means, but with the right attitude and purpose of mind I believe that you can get closer to the truth.
You are right. The accuracy of the device is completely unstated. This point stimulates me to make two related points
Maybe someone with an appropriate SAAB qualification can shed some light.
Hope the above is clear. Adrian
I had a physics teacher at college that made things very clear about precision and accuracy, particularly with respect to digital wonders. This notion that digital readouts are more accurate is complete bollocks anyway when the analogue device could be more accurate.
He loved fractions as the decimal equivalent of a recurring number for example is only an approximation. :)
David.
David, Since my last post, I turned on the telly. Right now, Rachel de Thame is on Call my Bluff and I couldn't give a toss about Precision or Accuracy. ;-)))))) Adrian
When measurements of heights are made to nearest centimetre, you're pigeonholing the data into classes, hence you can't infer any probability of a height of 123.5 because such measurements are not defined. What you have is a discrete distribution on the classes.
This is not the same as saying that the number of display digits is inappropriate. Clearly, the digital display takes out the subjective conversion into numbers, hence we can get a small edge by keeping the numbers intact, even when we know that the individual measurement error is larger. Reporting a single readout with all the digits may be meaningless, but meaningful if it is used in a statistical sample.
I suspect that most of the error is bias. If that's the case then relative measurements may still be useful for showing the results of e.g. changes to a particular car rather than comparing two cars.
Oh. Okay. Sorry. Must try harder.
-- Andrew Stephenson
Is she some hot totty? Well I just checked with images.google.com and you're excused.
:)
Yup, B- please resubmit! ;)
It's true, it's not exactly a major fault or anything, but it is interesting if you're a bit of an instrumentation nerd.
The problem won't be with the computer - that just needs timing accuracy which'll be measured in PPM rather than percent. The speedo measurement will cancel out because it's used in both manual and computer calculations. That leaves us with the petrol station vagaries (for the manual calculation) and the assumptions the computer has to make about fuel flow through the injectors. Any filling foul-ups should become apparent at the next visit to the money pit so, if the difference between the manual and computer figures is consistent, we can assume it's the computer that's wrong and it's down to the assumed fuel flow rate being wrong. Anyone still awake and reading by this point will twig that this means the SID is actually, in a really roundabout way, telling us something worthwhile(!) - the fuel flow through the injectors is different from SAABs expected figure. Others have said their readings tally up, so it looks like SAABs figures are reasonable and there must be a difference with our particular car. Who knows, correcting the error may actually improve the fuel consumption for real ;o)
Colin.
I read that as exhausted...
Colin.
One of the most wonderful things I once loved about my 9000 was the dash computer.
I was struck with the true distance variable you note when I actually started really using the dash CPU as outlined in the manual while traveling around the city - 'it, or something under the hood, just had to be wrong' - on several distance and time calculations.
However - I then did a series of direct 1,800 kilometer hops to and from central Canada on the TransCanada and was even more deeply surprised about how accurate it suddenly became over relatively straight tankfuls of 4 lane on cruise or off.
My take is that it's not a digital problem, it's physical variable that can't be gauged while the machine is measuring driveline speed on the bell housing. It simply can't accurately correct for things like wheel spin, clutch/torque converter slip and the amount of extra wheel travel taken up turning corners - you know - over time. The system, as advanced as it is, reads upstream of true differential output, which changes the distance a car/tire actually travels quite a bit relative to the engine management system - and over surprisingly short distances. It's the same thing that makes everyones three spokes fall out of lock step over time.
My old KZ-Z1R Rickman sled reads the true wheel travel extremely accurately given that it is driven from a gear on the front wheel itself. I can accurately flip to the reserve dimple based on the trip clock or time averages as I treated her to a factory calibrated police issue tac/speedo when I cleaned her up. I did the run a few times on it and it came within 1% on the highway as read by the stock SAAB system.
Anyway, it has since been my opinion that SAAB trip/dash electronics were driven by SAAB avionics logic - which very accurately corrects for everything but slide-slip - which is/was corrected by advanced GPS. But - as traveling through the air is not the same as rolling wheels down a road
- they kinda missed a step by not correcting the data stream using the ABS tickover as a variable. I guess it would have screwed the computer control system completely when the system ABS lights up and never gets properly repaired though.
Anyway, I often find myself with about another 20 kilometers in the tank after a tank of city driving - but - when I'm on a long run (like four or five highway tankfuls in a row), the system is pretty accurately tripping the pump light.
I've taken to checking inspection stickers for last official date on the actual pumps on the other point. Never put down to malice what can easily be explained by 'service rationalization' as they say.. :/ ..
How come?
That is not an error then. You are measuring different things, and thus cannot tell anything about accuracy of your computer.
That's right. However, gas pumps are fairly accurate and inaccuracy of pump does not really matter if you measure over longer period of time & distance (large number of fill-ups), because errors average out.
What I did was that for over 10tkm I recorded every single deciliter of gas I put into tank. I also reset computer at fill up but nowhere else, and I recorded also those readings. The rest was simple math. Computer and gas pump agreed within couple percents at every fill-up, and errors averaged resulted to less than 1%. Which means error has no offset and just a small deviation.
BR:Z
Oh, I knew that. I thought there were some other way to really reset the computer, not just the readings.
BR:Z
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