Computer fuel figure accuacy - Civinfo
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post #1 of 26 (permalink) Old 27th June 2007, 14:38 Thread Starter
 
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fuel figure accuacy

I know this is geekish but thought i would ask.

Has anybody worked out there average M.P.G. manually?
When i got my first honda i used to keep a note of my M.P.G. to see if it reached the figures quoted in the blurb but never got close (had a heavy foot at the time which didn't help).
When I got my new civic which is now 15 months old I still manually work out the figures and have noticed that it's always about 3-4 M.P.G. less than what the computer shows.

Just wondering if it my computer or if anybody else had noticed the same and maby its been calibrated that way so it looks as though the car is giving a better M.P.G. than it realy is.
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post #2 of 26 (permalink) Old 27th June 2007, 14:43
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I think this is a general problem with the computer on the Civic, not just your car.
Mine reads about 4-5mpg high, but my last car also told the same porkies.
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post #3 of 26 (permalink) Old 27th June 2007, 14:59
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I've had my 2.2 CDTI SE for a month and 1400 miles, filled up 5 times and checked MPG with a calulator. Computer says 58-60 MPG , calculator says 53-55. Still very good fuel consumption for performance! Still 5MPG better than a VW TDI PD...
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post #4 of 26 (permalink) Old 28th June 2007, 20:34
 
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I have been working out my M.P.G. manually since I got the car and keep the information on a spreadsheet. Over 6000 miles find the computer is over reading by an average of 10%.
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post #5 of 26 (permalink) Old 28th June 2007, 21:31
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I'm sure others will disagree but I am convinced it is deliberate, to make things look better than they really are. What a pity that such a useful reading is inaccurate, but then I guess you can always just subtract 10%.
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post #6 of 26 (permalink) Old 28th June 2007, 21:42
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I´m in on the deliberate theory...



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post #7 of 26 (permalink) Old 28th June 2007, 22:09
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honda over read everything by 10%.mpg,mph,
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post #8 of 26 (permalink) Old 28th June 2007, 22:22
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There is a theory that the speedos are calibrated to be 'fast' on all cars to prevent the manufacturers being sued for cars being caught speeding due to a speedo under reading.
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post #9 of 26 (permalink) Old 30th June 2007, 17:02
 
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My 1.8 seems pretty accurate with the fuel economy reading. I filled it to capacity (50 litre tank) when I took delivery and yesterday I refilled it to capacity after covering 588km. It took 46 litres of petrol to refill.

(46 litres/588 km)*100 = 7.823 litres/100 km, which is pretty much exactly what my civ's computer said - 7.8L/100km (36.2MPG in brit-speak ).
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post #10 of 26 (permalink) Old 30th June 2007, 20:53
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Interesting, better for Honda if it over-reads, makes them look better. Fortunately Civinfo members are smarter than that! Seems to be a common problem across the board and one I can see becoming more prevelent as things turn greener over the next few years. Maybe it will end up like odometers where they have to be as accurate as possible for legal purposes.
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post #11 of 26 (permalink) Old 30th June 2007, 23:51
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Fuel computers are always about 3-5 p cent out. I have always manually calculated my mpg as all petrol is logged in my little book and the relevant mileage as well so that I have a complete record of mpg throughout the life of the car.
Overall average mpg so far over 1800 miles is 39 compared to computer figure of 41 (1.8 ES i-shift).
Previous car was Jazz Auto overall average was 43 compared to 46 computer figure.I always go from full tank to full tank for maximum accuracy

Last edited by jazzman; 30th June 2007 at 23:53.
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post #12 of 26 (permalink) Old 2nd July 2007, 10:15
 
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Quote:
Originally Posted by GerryJ View Post
I have been working out my M.P.G. manually since I got the car and keep the information on a spreadsheet. Over 6000 miles find the computer is over reading by an average of 10%.
I'm also a geek, and have, as you, found the computer readings 8-10% optimistic over the first 8000 km. Shall we say 9%?
I'm also sufficient geekish to wonder if the computer is constant 9% wrong or if some conditions (urban/ extra urban) makes it less accurate than others?
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post #13 of 26 (permalink) Old 2nd July 2007, 10:39
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One thought.
Many fuel computers (I think also the Civ´s) also calculate "remaining distance on current fuel in tank". Could it be to make sure that you REALLY don´t run out they add some margin?



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post #14 of 26 (permalink) Old 2nd July 2007, 10:45
 
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Yes it does. And have plenty safety margin. I have never been below 5 liters remaining petrol - after the "honda string ensemble" playing for me for ages.
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post #15 of 26 (permalink) Old 2nd July 2007, 13:59
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Geek No3 here............

Spreadsheet method used here also produces different results.

My trip comp usually reads 44mpg average as my weekly journey rarely changes. I always put in the fuel 'till it clicks' each time and using the calculation produces either 39mpg or 40mpg.

Not being a techie could this be as a result of the over reading speedo = duff calculation or would that throw it the other way ?

I do have another theory:

I wonder if it actually takes into account when you are stopped? I have noticed in some jams that the mpg is not reducing whilst stationary, but if you start to crawl it does start to reduce!
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post #16 of 26 (permalink) Old 2nd July 2007, 14:24
 
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I have noticed over my 3,800 miles that the 'miles available on the remaining fuel' drops faster than the fuel is used.

My weekly journies are pretty much constant. When I fill up it tells me well over 500 miles to go, but by the time I am running out, the total of miles covered plus miles remaining is nearer 440.
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post #17 of 26 (permalink) Old 2nd July 2007, 15:39
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Quote:
Originally Posted by dondon View Post
I have noticed over my 3,800 miles that the 'miles available on the remaining fuel' drops faster than the fuel is used.

My weekly journies are pretty much constant. When I fill up it tells me well over 500 miles to go, but by the time I am running out, the total of miles covered plus miles remaining is nearer 440.
yep, mine still does that after 20K miles
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post #18 of 26 (permalink) Old 2nd July 2007, 19:33
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Quote:
Originally Posted by Kremmen View Post
Geek No3 here............

Spreadsheet method used here also produces different results.

My trip comp usually reads 44mpg average as my weekly journey rarely changes. I always put in the fuel 'till it clicks' each time and using the calculation produces either 39mpg or 40mpg.

Not being a techie could this be as a result of the over reading speedo = duff calculation or would that throw it the other way ?

I do have another theory:

I wonder if it actually takes into account when you are stopped? I have noticed in some jams that the mpg is not reducing whilst stationary, but if you start to crawl it does start to reduce!
Hi
That`s because all these computers in all cars measure the amount of fuel passing through the engine and part of the programming converts it back to MPG in readable form. The more time you spend sitting in traffic jams then the more inaccurate the figures will be.
The only way to calculate accurate figures is to do what I do and only fill up full tank to full tank and I write in my little book amount of petrol put in and at mileage covered
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post #19 of 26 (permalink) Old 3rd July 2007, 07:54
 
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Yes, that's when I noticed the difference between the computer and the world.
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post #20 of 26 (permalink) Old 5th July 2007, 11:20
 
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My daily commute has almost no time spent in stop-start traffic and the trip computer seems to be very close to the manually calculated consumption.

i.e - last fill up was after 466 miles
took 45.9 litres, = 10.1 gallons
466/10.1 = 46.1mpg

trip computer said 46.2mpg
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