Hi - I've noticed over the last couple of weeks that I've been reading these fora that there's a lot of suspicion re. over optimistic fuel consumption read outs.
Why is this? How big is the margin of error? Am I missing something very simple ie. the computer doesn't log fuel used on tick over when stationary?
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The margin of error varies by car maker, and sometimes even then it varies by car and use. For example mine (laguna2) varies from +10% to almost dead accurate. On a large ratio of motorway miles per tank its very accurate (in the order of 1mpg)
There is no single explanation, varies from how the number is calculated. Dont forget its only an average over the number of miles since the last reset.
There is only one guaranteed measure, and thats the amount you put in over the miles you do.
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There is only one guaranteed measure, and thats the amount you put in over the miles you do.
Assuming of course that the milage readout is accurate!! :)
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Ah but then you'll never know! It does seem that most cars milage readout is failry accurate. (unless you have a Pug of course that clocks up miles for each time the door is opened and closed.)
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Seems to me they are accurate on long run. If you spend alot of time stationary ( so the constant reads 0mph)then this is where the margin of error accumulates. Also aircon will affect it. I have found my own calculations match the manufacturer official figures (toyota) for the urban and combined cycle. The computer is accurate for extra urban and obviosly depends how much 'extra' one gives it on a long distance cruise
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The VAG onboard computer does a variety of readings, including average and current (reading from five seconds earlier) for both single and multiple trips depending on which MRI is in use.
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What\'s for you won\'t pass you by
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Seems to me they are accurate on long run. If you spend alot of time stationary ( so the constant reads 0mph)then this is where the margin of error accumulates. Also aircon will affect it. I have found my own calculations match the manufacturer official figures (toyota) for the urban and combined cycle. The computer is accurate for extra urban and obviosly depends how much 'extra' one gives it on a long distance cruise
How will static running or use of Aircon affect the accuracy of the on board computer? It will only affect consumption.
If you use a gallon of fuel with a combination of static running, use of aircon and travelling 20 miles the computer will correctly display a consumption of 20mpg. Less static running and aircon use might give, say, 30mpg.
There have been a number of threads on this subject and IIRC there was no set pattern; some were optimistic, some pessimistic. My VW is optimistic by about 3%(1mpg) SWMBO's Toyota gets about 1-2 mpg than the computer reading.
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As has been said, this question has been raised here before. When accusing computed consumptions of inaccuracy, we must remember all possible sources of inaccuracy: either in measuring the distance covered or the fuel used.
However it is done, the odometer reading will be the basis of calculation unless some enterprising driver tows his own measuring wheel behind. So to get absolute values, you should calibrate your vehicle against the 100-metre M-way markers (this has also been discussed here) and correct for distance inaccuracy.
'Manual' calculation uses volumes measured at the pumps, which may vary slightly, but accuracy is likely to be high due to the attentions of Trading Standards officers. Your car's computer can't tell how much fuel you add, and the tank sender is nowhere near accurate enough to monitor the fall in fuel level, so the computer has to accumulate the quantities dispensed by the fuel injection system. I suspect this is quite accurate once the engine is warmed up, but less so otherwise, leading to the discrepancies mentioned here. So a 'motorway' car may calculate close values while a local runabout might not.
That's my opinion FWIW.
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I find the computer in my Mondeo to be very accurate. It also switches to litres/hour when stationary and this does affect the average economy reading.
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I don't think the average fuel consumption shown on my Focus computer can be very accurate. It indicates to 0.1 mpg, but the actual reading is invariably either something point 2 or something point 7. Can't be a coincidence.
Instantaneous fuel consumption changes so rapidly that it hardly means a thing.
Outside temperature is useful.
Mileage remaining in tank is useful.
Average speed is merely interesting.
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L\'escargot by name, but not by nature.
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Mileage remaining in tank is useful.
I wonder how this is worked out:
Using your average MPG since last reset? or over the last, say, 5 miles?
Also it cannot calculate accurately how much fuel you have put in, so I can only assume it must use the float in the fuel tank which is very hit and miss.
I had this feature on a couple of cars and whilst I agree it could be very useful, it was so inaccurate to be useless. As the tank approached empty it would drop its estimate by 20 miles or more in a mile or so. On a Primera GT I once drove 50 miles after it showed empty(I had a can in the back)
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I find the reading showing distance left is very useful. At least it was on a company Vectra SRI that I had as it was so accurate. It ran out of petrol twice at exactly the point it said it would (1 mile after the reading went to 0) and in one case I coated to a halt on a garage forecourt! That was a close one (I had forgotten there were no services on M11!)
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I believe (with no technical knowledge whateve) that fuel gauges / trip computers always have a "safety factor" buit in as running out of fuel on most modern cars, certainly modern turbo diesels is an expensive business.
Even running down to single figures of miles left on the trip computer I have never managed to get more than 14 gallons in to what is supposed to be a 15 gallon tank, but I've never pushed it past that to see.
For real excitement try using a vehicle without a functioning fuel guage, the one on the Commer hasnt worked for about 8 years.
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The laguna "miles to go" is obviously worked out using the fuel tank level sender. If oyu top it up " the miles to go " suddenly becomes more.
Usually the low level warning light comes on with about "70 miles to go" and when the wafrning light comes on the "miles to go" goes blank. So the last 70 miles is guesswork.
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