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fuel figure accuacy

Computer 
4K views 25 replies 17 participants last post by  FergusTheDog 
#1 ·
I know this is geekish :oops: 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.
 
#12 ·
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%? :p
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?
 
#8 ·
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.
 
#9 ·
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 :)).
 
#10 ·
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.
 
#11 · (Edited)
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
 
#13 ·
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?
 
#15 ·
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!
 
#18 ·
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
 
#16 ·
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.
 
#21 ·
One of the points I was making is that less stop start motoring then more accurate the figure and also it`s interesting to keep your eye on the "B" figure on the computer readout which gives the total overall figure over your total mileage:D
 
#23 ·
I am very pleased to here that there are others out there with little black MPG books!

I was starting to believe my filling statition ritual was as abnormal as my partner suggests!

I have 2 books, one for the Civic and one for her Jazz.

Find monitoring MPG easy way of checking a cars health, small consistant drops in MPG can give early indication of a sticking brake, low tyre pressure or make you double check that the air cleaner was changed a the last service.
 
#26 ·
Yes, I do a quick calculation each time I fill up. I tend to work on the simple sum that 45.4 mpg equals 1 litre for each 10 miles travelled and work from there. On that basis my diesel is doing about 49mpg in reality and shows 48.9 on the computer. So I'm very happy.

Got into this habit with my previous car which did about 34-36mpg in reality but reported 44-47mpg through the computer. Guess which figure the dealer believed and quoted? Quite fun to see them claiming 45mpg and then putting 70 litres in after 400 miles in their tender care.
 
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