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balls-out
Original Poster
1,397 posts
100 months
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I frequently see people saying how many mpg they get based on the car computer.
Do people check their accuracy. Can help thinking that it might be in a manufacturers interest to 'calibrate a positive message', so that people thinks they are getting better mog than they are.
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Honestherbert
305 posts
16 months
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All the ones I have used have been very,very optomistic! particularly the ones in BMW M cars!! but then thats just like their power claims 
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Roo
5,887 posts
76 months
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The one in my Mustang is extremely accurate based on mileage between fill ups etc.
The one in the Prius over reads by about 10%. As does the speedo and mileometer.
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mike9009
1,230 posts
112 months
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Roo said: The one in the Prius over reads by about 10%. As does the speedo and mileometer. I suspect this is where most the inaccuracies come from. Speedos (for safetys sake  ) always over-read so the user is not in danger of breaking speed limits. I would suggest that this means the consumption computer always 'thinks' a greater distance has been travelled (assuming it uses the same data source) and therefore gives a better MPG figure..... Mike
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CraigyMc
5,068 posts
105 months
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The one in my BMW 320d/ED is about 2mpg pessimistic.
I do full-tank to full tank calculations every time as well as resetting the BMW fuel economy gauge every time.
C
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blank
1,791 posts
57 months
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In my experience, most cars tend to be up to 10% optimistic.
However my current car is always within 0.1 mpg of a 'proper' calculation.
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kambites
32,864 posts
90 months
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Our Skoda's one seems to be as close as I can realistically measure. Certainly within 1mpg.
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Pixelpeep
822 posts
11 months
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i hope mine isnt being optimistic.- it rarely reads over 17mpg!
I did a rough spreadsheet and it calculated over a month my car actually did 11.1mpg - i refused to believe it, sighting my obvious excel failings.
maybe it was right!
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sparks_E39
6,575 posts
82 months
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My E39 reads about 2 mpg over.
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NateWM
1,220 posts
48 months
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Most do seem fairly optimistic. My old ST220 was bang on though. Said I had 0 miles left half a mile away from the petrol station. As I pulled into the forecourt, it started to splutter and cough. Close call eh? 
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otolith
19,408 posts
73 months
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It's conceivable that the algorithm being used by some cars to calculate the displayed "average" fuel consumption is doing something other than adding up fuel injected and dividing by miles travelled since last reset.
Edit - reciprocal, obviously.
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tonker
43,849 posts
117 months
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Golf - about 1.5% overread.
BMW - about right
Triumph bike - reads about 4.5% under....
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kambites
32,864 posts
90 months
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Always struck me as odd that people are intensely distrustful of their ECU's MPG reading, let trust the odometer when doing manual calculations.
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PanzerCommander
2,703 posts
87 months
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Roo said: The one in my Mustang is extremely accurate based on mileage between fill ups etc.
The one in the Prius over reads by about 10%. As does the speedo and mileometer. I find this with the 'Stang too, but then if like other posters have said that speedo accuracy comes into it then that might be why, I find that when comparing to the GPS speed measurement the 'Stangs speedometer is saying 70 when the GPS is saying 69.
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A900ss
452 posts
21 months
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Over 7k miles in my 520d, it over claims the MPG by 6%
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Engineer1
7,183 posts
78 months
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kambites said: Always struck me as odd that people are intensely distrustful of their ECU's MPG reading, let trust the odometer when doing manual calculations. Haven't people "read" the speed at the OBII port and seen it matches the sat nav while the speedo is showing 10% higher. The Odometer would be odd if it miss read by as much as distances between motorway junctions etc would soon show it up, print off a route plan then compare the route and your mileage.
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s3fella
6,487 posts
56 months
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blank said: In my experience, most cars tend to be up to 10% optimistic.
However my current car is always within 0.1 mpg of a 'proper' calculation. Except you may use the car odometer to take the mileage between fills so your proper calc will be out by whatever the odo is out by.
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DuckDuck
263 posts
17 months
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My car reads 1.2 mpg pessimistic. In the 5 series (E60/61) you can calibrate the car to actual mpg yourself.It's a bit of a faf but I got it near enough.
Duck
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Roo
5,887 posts
76 months
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PanzerCommander said: I find this with the 'Stang too, but then if like other posters have said that speedo accuracy comes into it then that might be why, I find that when comparing to the GPS speed measurement the 'Stangs speedometer is saying 70 when the GPS is saying 69. They do seem to be incredibly accurate. Must be an American thing as they all seem to be like it.
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Ari
7,205 posts
84 months
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I have an iPhone app that records mpg on each fill up.
Interestingly my last SLK (2005) was about 10% optimistic, whereas my current one (2009) is absolutely bang on.
Good comments about accuracy of milometer, but checking speedo against GPS it's very close, so be surprised if mileage wasn't given that presumably they use the sane data.
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