Accuracy of fuel consumption on mfd

[Redalltrack]

Apologies if this has been discussed previously.
I am a happy owner of a mk7 golf and find the various stats indicated in the mdf quite useful - particularly the ave mpg figure.
From reading various posts, it appears that some owners treat the mdf figures as gospel. Although not normally a stickler for recording such data I decided to do a few brimful to brimful checks to ascertain the accuracy of the mfd figures and satisfy my curiosity. I have found the mfd figure to be 5-7% optimistic which I do not think is too bad. I have had other makes of car in which the mdf figures have been have so inaccurate to render them useless.
I assume this discrepancy can be adjusted in the VCD's. As my car will be having a service shortly I was wondering if I could ask the VW dealer to recalibrate the mfd accordingly or is this a complete no no!
Any advice/experience would be appreciated.

[Watts]

I've checked mine a couple of times and found it to be spot-on! I was rather surprised. So now I do take it as gospel.

[fredgroves]

Even your "brim" definition is variable, unless you truly fill it until you can see it overrun the filler.

I'd have also thought that temperature variations could also account for differences in volume too...

[Daz Auto]

Even your "brim" definition is variable, unless you truly fill it until you can see it overrun the filler.

I'd have also thought that temperature variations could also account for differences in volume too...

This is a myth. The fuel underground is at a relatively constant temperature. So it makes no significant difference.

If it is a diesel, it is really hard to fill to the brim with the new mis-fuel device fitted.

I did a few calculations on my petrol car and found I was getting 35mpg. That's what the computer said too. So yes, it appears quite accurate.

It is funny watching all the members of an Audi forum bang on about how economical there Audi S3's are. Some of them actually believe their cars will do 40mpg because the computer says so. Most of them are getting around 350 miles to the tank which is actually around 30 mpg. "but the computer says..." I told one guy to try and drive 12x40 = 480 miles in his S3 on one tank and let me know how he got on.

This week , the fuel light had not come on and I put 47 litres in the car (not to the brim). The manual says 5lt fuel reserve. So I must have at least a 52 ltr fuel tank

[dubber36]

This week , the fuel light had not come on and I put 47 litres in the car (not to the brim). The manual says 5lt fuel reserve. So I must have at least a 52 ltr fuel tank

This assumes the pump is accurate.

Many years ago there used to be just one fuel station in our little town. He could virtually name his price. A supermarket opened up and started selling fuel much cheaper. He dropped his prices to match, so I went to him and filled my Mk4 Golf up. That has a 55 litre tank, but according to his pump, I put over 70 litres in it.

[fredgroves]

This week , the fuel light had not come on and I put 47 litres in the car (not to the brim). The manual says 5lt fuel reserve. So I must have at least a 52 ltr fuel tank

This assumes the pump is accurate.

Many years ago there used to be just one fuel station in our little town. He could virtually name his price. A supermarket opened up and started selling fuel much cheaper. He dropped his prices to match, so I went to him and filled my Mk4 Golf up. That has a 55 litre tank, but according to his pump, I put over 70 litres in it.

Thats highly illegal under the Weights and Measures Act:

http://www.coventry.gov.uk/info/30/trading_standards/350/weights_and_measures

[I wanted a GTi]

I tried mine when I got it and it was between 1 and 2mpg out.

[itavaltalainen]

Mine is accurate to around 2-3%.

[robtt]

I check mine brim to brim  most fill ups and the read out is very accurate .

[Gnasher]

Another thing to think about is the effect of having a tuning box fitted - these allegedly make the fuel computer more optimistic.

Never checked mine against it - I don't think I could have handled the disappointment!