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Discussion Starter #1
Hi guys, don't know if this has been talked about, can't find it if it has.

My 53 Extreme is reading 41.5mpg, this has averaged out over some 2000 miles now and is pretty constant. I used to reset it every week but it changes too quickly and doesn't give a true inpression of average mpg over time, bouncing between 37 and 44mpg all the time, so I've started just resetting my trip computer on fills and nothing else.

The thing is the mpg estimate of 41.5 just doesn't compute. I put £30 worth of diesel in a week, alternating between the BP normal (so I get 33ltrs) or the Ultimate (where I get 30ltrs).

On the 33ltr fill I average 340-360miles, depending how I drive, and with the 30ltr fill I get around 330-340miles, but I've never had less than 330 miles for 30 litres.

Well, at 4.54 litres per gallon this works out at 30l/4.54l = 6.6 Gallons, 330m/6.6g = 50mpg.

or: 33l/4.54l = 7.2 gallons, 360m/7.2g = 50pmg

So the figures are relative, but how the hell can the computer be so far wrong? It's a simple calculation - what's more, it even estimates how much mileage is left in terms of fuel when I fill up correctly (usually within about 10miles either way), so if it knows how far I'm going to get on the fuel I've put in, how can it work out the mpg and be so wrong?

Anyone shed any light on this?
 

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Do you also reset the mpg trip counter? it may be you're still getting a total avg.

On our Lag II there is a seperate trip counter for the MPG (next screen after MGP)

Tom
 

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Fuel computers ain't worth the money - I my opinion they are at best only an estimate - with an accuracy of approx plus or minus 10%. In all honesty they're about estimating and without accurate measuring devices how can they calculate the amount of fuel used. They just seem to another bell or whistle added on to impress the buyer. In all honesty the only accurate method of measuring fuel consumption would be to use a specialised flow meter and I have'nt heard of one yet being fitted to a car produced for the public. In all honesty I wouldn't rely on them. Fuel consumption needs to be measured over a broad range of circumstances as there are just too many variables such as driving style, weather, road conditons, traffic, type of road surface, tyre pressure, etc., (the list goes on) to take into consideration. The best way is to fill her up to the throat on each fill and then calculate the average over about 3 tank loads. Also I'm of the opinion that the computers are set up to give the best results as otherwise it would affect sales. Think about it.:rolleyes:
 

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I was shown over 50Mpg in my 1.8 Lag 11

This was going down a long hill off the throttle in 5th gear. LOL

I never trust my fuel computer.
 

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Discussion Starter #6 (Edited)
....In all honesty they're about estimating and without accurate measuring devices how can they calculate the amount of fuel used.....

They have extremely accurate measuring devices; the ECU and common rail injection system is precision tuned, you're not telling me it doesn't know exactly how much fuel it's using, couple that with total distance travelled and then it's just a very simple calculation. It knows how far I've travelled, this is a journey I've made twice every day for the last 2 years so I know how far it is, it 'estimates' this bang on. It even estimates how far I'll get when I fill up, it'll read 440 miles, or thereabouts, I then drive 330 miles in a week, the estimate usually reads around 100 miles left (give or take 10-15 either way depending) but the average mpg is 41.5 constant, actually it's just gone up to 42.1 this week - which is odd because diesel just went up!

So, if it can estimate the fuel remaing (in terms of miles) and it knows the distance covered since filling up, how can it not know the mpg? :crazy:

Also I'm of the opinion that the computers are set up to give the best results as otherwise it would affect sales. Think about it.
So surely it would read more than I was getting in this case, not less? I get approx 46-50 but it says I'm getting 41.5 (well, 42.1 as of this week).

I know it's screwy, I just wondered if it was common among the model - my guess is it'll be a software error as the sensors seem Ok. It's not km/m conversion because I'd be better off, not worse.

Thanks for your input though guys, it's always appreciated and your opinions valued.
 

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They have extremely accurate measuring devices; the ECU and common rail injection system is precision tuned, you're not telling me it doesn't know exactly how much fuel it's using, couple that with total distance travelled and then it's just a very simple calculation. It knows how far I've travelled, this is a journey I've made twice every day for the last 2 years so I know how far it is, it 'estimates' this bang on. It even estimates how far I'll get when I fill up, it'll read 440 miles, or thereabouts, I then drive 330 miles in a week, the estimate usually reads around 100 miles left (give or take 10-15 either way depending) but the average mpg is 41.5 constant, actually it's just gone up to 42.1 this week - which is odd because diesel just went up!

So, if it can estimate the fuel remaing (in terms of miles) and it knows the distance covered since filling up, how can it not know the mpg? :crazy:



So surely it would read more than I was getting in this case, not less? I get approx 46-50 but it says I'm getting 41.5 (well, 42.1 as of this week).

I know it's screwy, I just wondered if it was common among the model - my guess is it'll be a software error as the sensors seem Ok. It's not km/m conversion because I'd be better off, not worse.

Thanks for your input though guys, it's always appreciated and your opinions valued.

What would be interesting to take note of is the gallons used figure and the miles travelled to see which one does not tally with reality.

Ian
 

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Discussion Starter #8
Good point, I missed that one, I'll bet that's exactly what it's based on and if it is then that'll be the problem. Thanks heaps Eeeps. ;)
 

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Hi Schmooster - how exactly can the ECU or common rail measure the amount of fuel passing throught it - or am I missing something here.:confused:
 

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Hi Schmooster - how exactly can the ECU or common rail measure the amount of fuel passing throught it - or am I missing something here.:confused:
I'm guessing here but given that the fuel injection system must have precise control over the fuel / air mix then it must 'know' how much fuel is being injected.

I suppose that rather than measuring this the injectors are precisely calibrated so that the ECU can 'know' how much fuel is being injected on every pulse.

Eeeps.
 

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I see your point - the ecu will define when an injector opens and for how long. But surely that does'nt actually measure the amount of fuel that has passed through. At best it would only be an assumption of what ought to have been used. I think there could be a big difference in the two. There are just to many variables for this to be accurate. In my experience the only accurate way to measure the volume of a gas or liquid passing a given point be it in or out is to use a flow meter (e.g. similar to whats used on the metering system on oil tanker delivering home heating oil). As yet I've never seen a metering system fitted to a bog standard mass produced car. The only recommendation I would make is to buy a packet of salt and take a good pinch when reading what the computer reading says.:d
 

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Discussion Starter #12
Absolutely, I'm just astounded it should be so inaccurate, begs the question as to whether other areas suffer the same degree of inaccuracy - anyway, it's only a guide. :)

Cheers Fellas. ;)
 

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Interesting debate, I'll just wade in here with my own figures,

Google Docs & Spreadsheets - scenic fuel cost calc2

Since I bought the Scenic last December I've been keeping a keen eye on mpg as every penny is a prisioner.

It was on another thread that someone mentioned how inferior supermarket fuel can be, I'd been using Tescos as it was usually the cheapest and clubcard points were racking up nicely.

I switched to see if there was a difference, not conclusive yet as only been doing it for a short time, my bog standard Scenic has nothing so flashy as a mpg computer so it's all done by filling to the throat.

I'm still intending to put some proper oil in as well, even going the amsoil route, when I do I'll make a note on the sheet when it happens.

Loving Google doc's :)
 

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I don't agree that the ecu has to know precisely how much fuel is flowing to set the fuel air ratio correctly. So long as it has a rough idea, the precise adjustment is done based on the Lambda sensor output (i.e. Lambda sensor says lean mixture; lengthen injector on time etc). The mpg computer must do its sums based on the standard calibration values of the fuel pressure, temperature etc. Any inaccuracies in these sensors will inevitably give an mpg error.

Now, to the reason I 'm visiting this thread! My V6 started misfiring last week and the ecu warning came on with "pollution monitor faulty".
Anyway, I found the fault - ignition coil on cylinder 2. After changing the coil, I reset the ecu fault codes with my laptop and obd connector and everything seemed fine.
Next morning on the way to work I looked at the "miles to empty" and mpg displays to find that both are just showing dashes. They worked fine before the misfire; pressing the reset has no effect. I'm lost, suggestions anyone?

Cheers
Andy
 

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Fuel calculation

Nice work Pyro - youv'e got the right idea - good old spreadsheets never tell lie and as you have rightly done you've calculated over a decent perod of time. In my opinion its the only way. As a matter of interest have you had any servicing or repairs done during the period of calculating. If no servicing has been done why not see what hapens if you do a good old simple oil change before going down the Amsoil route - it would be interesting to see if the benefits out weighed the overall cost. Well done!:) I have set up a spreadsheet where you can calculate overall running costs to include, depreciation, insurance tax, etc. At the end of the day it's the overall cost per mile which really counts.:)
 

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I don't agree that the ecu has to know precisely how much fuel is flowing to set the fuel air ratio correctly. So long as it has a rough idea, the precise adjustment is done based on the Lambda sensor output (i.e. Lambda sensor says lean mixture; lengthen injector on time etc). The mpg computer must do its sums based on the standard calibration values of the fuel pressure, temperature etc. Any inaccuracies in these sensors will inevitably give an mpg error.

<snip>

Cheers
Andy
So that's what the lambda sensor does.
Is there one of these per cylinder? if not how does the ECU know which injector on time to lengthen?
My point is that the injectors need to be calibrated or atleast matched (to a degree). Can one injector be replaced or must you alway replace them together to ensure they are still matched?

Ian
 

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Hi Eeps - as far as I know mass produced cars don't have a Lambda sensor for each cylinder and usually rely on one or possibly two to measure the exhaust gases. And yes ideally all injectors should be replaced at the same time - just like you should renew all the spark plugs and coils similtaneously.

P.S. Does anyone actually know what data the on-board computer or the ECU gathers to make its calculations or are we just guessing.:)
 

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Discussion Starter #18
I'm beginning to think it's tied in to the average speed calculation.

Technical data will specify a return of 48mpg based on a steady 56mph for example, so I'm guessing it translates the average speed into consumption from a reference table, and nothing more - this would explain it's innacuracy but it would also give a truer average consumption as well as it's not just about the distance you've covered, it's about how quickly you've covered it - I don't think it can gauge fuel consumption this accurately which is why the figures don't tally.

Tomorrow I will have a new theory, and eventually we will stumble across the answer. Sadly we'll probably be completely unaware of the fact, or at the point of not particularly giving a monkeys cuss any longer, either way, it's fun to speculate. :)

1 rep point to anyone who's theory involves naked women and natural yoghurt. :devil:
 

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I think that if you get a naked woman to rub natural yoghurt over the dashboard it will revert to what you expect to get.lol.....Ever since my Scenic went in for a recall to do with updating the ecu I only seem to be averaging 40.1mpg...dunno if that has anything to do with it. By the way it was only serviced less than a week before the recall and the air and pollen filters were changed (supposedly)
 
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