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Discussion Starter · #1 ·
I did my 6th fill up of Dafny today. The trip computer reading for the whole tank was 56.6mpg over 350.4 miles since the last fill up (brimmed)

However it took 29.93 litres of petrol to brim the tank which equates to just 53.2mpg, a full 6% lower than the computer.

The previous fill up was a massive 8% under and every previous fill up has been out by between 4% and 8%.

Previous experience in numerous other cars including VW and BMW company cars suggests this is common however some people are claiming the system is accurate?

Any suggestions as to why mine is so far out? My average speed seems much higher than other people because most of my driving is on motorway. The computer did seem the most accurate when my average speed was lower. My tyres are running eco pressures. Dafny is the 1.4T 6 speed with 215/45R17 tyres.

Overall I'm still very impressed by her economy but I would like to know why the trip is so far out on Dafny when others reckon theirs is accurate.
 

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Discussion Starter · #2 ·
I've done some googling about this and just about every owners forum is reporting same thing, pretty much every fuel computer across all manufacturers read 'optimistic' by as much as 15%. It seems Vauxhall is actually amongst the more accurate! It seems they never read pessimistic so the assumption is that the manufacturers are trying to 'inflate' the mpg of their vehicles because the know that most people will never verify the figures.
 

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I have read this on the internet too about the computers being on average 7% out.

I am in the process of working my figures out but I haven't emptied the tank yet.

I did a 300 mile trip over the weekend and got 57mpg out of the car, I cruised at 65mph for most of it. Much better then the 49.8 I got on my previous 300 mile trip 2 weeks ago at 70 - 75 mph.

When I get the figures I will let you know the results.
 

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Discussion Starter · #4 ·
Thanks Reddaz. Yes mine is averaging about 6% lower than the trip computer states when I work it out by miles travelled over litres used since last fillup. I know there will be some innaccuracy between different petrol station pumps etc but as I am brimming the tank each time the difference in fuel should be minimal and certainly would not be as a big as 6% error!

For example if I put 34 litres in and it was a full litre short of a full brim (compared to the last brim) in the filler tube (unlikely) the result would still only be 3% out.
 

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When I filled up my car the orange light was on and last light was red, I drove about 25 miles with it on before filling up. To get it full the tank would only take 39 litres, meaning I still had 6 litres left in the car! I have a feeling the last light on the car (before the orange light comes on and last light turns red) would get you 100+ miles because when the last light is left the computer still says on average 140 miles in the tank! I haven't had the balls to risk it though!
 

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Discussion Starter · #6 ·
Yes, daparky managed to drive until he was on vapours and he had to fill up with 46 litres!! :eek:

However it doesn't really matter how many litres you fill up with. As long as you brim the tank, drive for a few hundred miles and then brim the tank again, your mpg should simply be calculated as follows:

miles done since last brimmed tank / (litres it took to brim again / 4.546)

eg. 360 miles / (34 litres / 4.546) = 48.3mpg.

On mine, every single fillup (of 6 now) has resulted in that calculation (and fuelly and other online mpg tools) giving lower mpg readings than the trip computer which I always reset at the same time as filling up. In every case the computer reads high too so it can't be temperature causing fuel expansion or contraction, or different pumps measuring differently because surely I'd get some readings that were lower or equal. They are always higher which suggests the manufacturers have fiddled the computer mpg figures slightly.

I did read however that on the turbo engines the 'optimistic' computer mpg is worse. This could be because the computer can't accurately measure the fuel used when on boost? The computers on normally aspirated engines seem to be more accurate according to what I've read.
 

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Since I've had mine remapped I've just been too tempted to use the power constantly meaning I'm averaging 25mpg and I can only run her on vpower :(
 

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mud4fun said:
Yes, daparky managed to drive until he was on vapours and he had to fill up with 46 litres!! :eek:

However it doesn't really matter how many litres you fill up with. As long as you brim the tank, drive for a few hundred miles and then brim the tank again, your mpg should simply be calculated as follows:

miles done since last brimmed tank / (litres it took to brim again / 4.546)

eg. 360 miles / (34 litres / 4.546) = 48.3mpg.

On mine, every single fillup (of 6 now) has resulted in that calculation (and fuelly and other online mpg tools) giving lower mpg readings than the trip computer which I always reset at the same time as filling up. In every case the computer reads high too so it can't be temperature causing fuel expansion or contraction, or different pumps measuring differently because surely I'd get some readings that were lower or equal. They are always higher which suggests the manufacturers have fiddled the computer mpg figures slightly.

I did read however that on the turbo engines the 'optimistic' computer mpg is worse. This could be because the computer can't accurately measure the fuel used when on boost? The computers on normally aspirated engines seem to be more accurate according to what I've read.
Hi Ian, think you got a Typo in your calculator should read: 360/ (34 litres X 4.546 = 48.13 mpg

Derek
 

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Discussion Starter · #9 ·
Derek, I'm sure it is correct. I'm dividing 34 litres by 4.546 to get UK gallons?

1 UK gallon = 4.546 litres

I have brackets around that calculation so it is done first then I divide the miles by the gallons to get mpg.

Edit: Sorry, did you simply mean my example shows 48.3 and should be 48.13? Yes you are right, typo :)
 
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It seems silly that the fuel computer isn't more accurate, after all the ECU knows exactly how much fuel it puts in as it controls how long the injectors are open and knows the fuel pressure, air pressure and air temp. The only variable that isn't accurate is distance covered due to the legal requirement for speedos to over read, tyre wear/pressure ( although the E does have a tpm system so it should be able to work out rolling radius). Sorry, just my mind working overtime again lol.
 

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Discussion Starter · #11 ·
Agreed Gary, it should be possible to get it accurate. Hence the reason many people have concluded that the manufacturers do it on purpose. Lets face it most people with an mpg readout on the dash will just believe what it tells them. Very few people do high enough mileages to worry enough about accurate mpg recording. A 6% margin of error is probably good enough for most.

BTW it can't even be due to the odo overreading because I'm using that same odo reading to work the mpg manually and it still shows the variance. It must be in the fuel measurement itself that the error is?
 

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My Trip computer says after 3000K ( Almost 1865 Miles ) and it says i have and average of 5.4L/100 ( Almost 52MPG )... but i have an average of 6.21L/100 ( Almost 45MPG!!! ), isn`t this a big diference? F$CK!

Thats a fail of 13,46%... How can it be?
 

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Discussion Starter · #13 ·
My computer over-reads by between 4% to 6%

Since I pumped my tyres up to the eco settings the error is now much smaller than it was (and mpg is also higher too) which suggests the computer is not correctly calibrated to the tyres or tyre pressure settings. Have you made sure your tyre pressure setting on the dash matches your chosen set of tyre pressures. eg. eco pressures in tyres you should select eco on load settings. It may be that the computer has a built in rolling radius setup for each tyre pressure range. Obviously increased tyre pressure means larger diameter tyre which equals under-reading of odometer. The computer may compensate for this?
 

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Discussion Starter · #14 ·
I've now done a few fillups and my actual mpg varies widely from the trip computer reading. I've started to add the trip readings into the notes on my fuelly fuel ups so I can monitor the differences. So far on the fuel ups that I've remembered to take photos of the trip I've got the following:

Trip showed 51.2mpg , actual is 47.9mpg - variance of 7% overreading
Trip showed 46.7mpg , actual is 45mpg - variance of 4% overreading
Trip showed 52.7mpg , actual is 49mpg - variance of 8% overreading
Trip showed 51.1mpg , actual is 48.6mpg - variance of 5% overreading
Trip showed 56.6mpg , actual is 53.2mpg - variance of 6% overreading
Trip showed 52.4mpg , actual is 50.2mpg - variance of 4% overreading
Trip showed 59.8mpg , actual is 56.5mpg - variance of 6% overreading

Curiously all the readings that were closest to actual where done on mostly low speed local journeys. The biggest variances were on much faster mostly motorway journeys. It seems the overreading on thr trip computer is related to average speed. The higher the average speed the worse the overreading?

One thing that is also clear from above is that the average mpg is gradually improving as the miles have piled on. Now at nearly 4K miles and the mpg is still improving despite my driving (and journeys) being largely unchanged. So despite the 4-8% overreading the car is still producing fantastic economy.
 

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Interestingly I've just worked mine out over the last two fill ups and my trip is UNDER READING! By quite a bit as well.

Trip 1 for the last tank was showing 38.3MPG
Trip 2 for both tanks was showing 38.2MPG

The actual MPG is coming back at around 49MPG. Both times the car took 43-44 liters so I'd imagine that to be a fairly accurate figure. Very impressed with that figure. I got around 470-475 miles each time with the light flashing and showing 15-20 miles range left!

Car has only just ticked over 3000 miles so if that improves a bit more I will be more than happy. My last car used to average 28MPG so it's a massive improvement.
 

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in the manual it states that the trip displays for the mpg is an average.
my 1.4 reads about 39mpg and i get about 470 miles out of a tank depending how i drive
 

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All the hypothesis aside, please be aware that if you brim your tank from empty (and I mean empty), you will not get more than 43 litres in it....on a Corsa or any other fuel tank irrespective of its supposed maximum capacity for that matter. Remember your Physics people.....
 

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Moog70 said:
All the hypothesis aside, please be aware that if you brim your tank from empty (and I mean empty), you will not get more than 43 litres in it....on a Corsa or any other fuel tank irrespective of its supposed maximum capacity for that matter. Remember your Physics people.....
I believe the fuel tank is actually 45 liters
 

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My point is that it is impossible to fill to 45 litres due to air content gap
 

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Moog70 said:
My point is that it is impossible to fill to 45 litres due to air content gap
That's very weird because I filled up with 44.04 litres of fuel only just today...?
 
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