OBC fuel consumption readings

mjt

mjt

mjt

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31 Aug 2007
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What are the chances of my average fuel consumption readout being wildly innacurate?

Apparently I've averaged bang on 30MPG on the last two tanks according to the OBC.
However, after doing some very simple maths, my calculations were 24.8 and 26.4 - from the same number on the av. readout :confused:

Would the fact it's a relatively small tank make it rather innacurate if I don't properly brim it until it's all over my shoes every time?
It seems unlikely as there'd have to be 5 litres of fuel between the tank and where the nozzle goes..
FWIW, I've done 46.66l / 410km and 41.14l / 383km.

Now that's nowhere near 30MPG unfortunately, and yes I did reset the OBC.
I've found a guide to correcting the readouts here, so I could just fiddle with that and see what happens..

I'm just surprised that the BMW one is so innacurate, considering my Leon was totally accurate every time.
 
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Always off a bit, do the whole brim, drive x miles then brim again and work it out.

I avoid watching my MPG thing too much, I know if I rag the car I'll get 28~, if I drive it normally 30~ and if I drive like a saint 32~.
 
Brim to first click, drive, brim to first click, count fuel used v miles driven = most accurate. If this method and your trip computer disagree, your trip computer is not correctly calibrated.
 
I suspect this is where I've been going wrong then.
Brim for me most certainly isn't first click. I'll try that next time I fill up, but that'd mean there's 5 litres between first click and properly full. (assuming the trip is correctly calibrated and first click is a full tank).

FWIW, it's on 1000 which is the default setting. Wait and see I guess.
Not quite sure how it works though (you can change the setting from 750 -> 1250)
 
It doesn't matter whether brim is one click or two clicks as long as its the same each time you do it. You just need to make sure the tank has the same amount of fuel in it after each fill, so you can calculate exactly how much fuel was used.
 
But that's the thing; how do you know you're putting in the same amount of fuel every time? Who's to say that first click is the same amount every fill?
Even using my method of putting in as much as I can to the nearest euro (OCD), the readout must be totally wrong considering it's saying I've used 5 litres less than I have..

Sucks though, this car's not as economical as it's making itself out to be :p
Not surprising though considering the average speed over the last 4 tanks has been 30 mph..
 
[TW]Fox;17847037 said:
Brim to first click, drive, brim to first click, count fuel used v miles driven = most accurate. If this method and your trip computer disagree, your trip computer is not correctly calibrated.
Click to expand...

Just brim it? First click could be caused by a splashback or something couldn't it?
 
[TW]Fox;17847037 said:
Brim to first click, drive, brim to first click, count fuel used v miles driven = most accurate. If this method and your trip computer disagree, your trip computer is not correctly calibrated.
Click to expand...

speedo is allowed to over read by 10%, which would give an inaccuracy.

my e30 is about 2.5mpg of using foxes method.
 
beermonster said:
What sort of driving are you achieving the 25mpg on, town, mixed or motorway.
Click to expand...
My commute now consists of 10-15 minutes of traffic jam (thanks roadworks!) followed by 10 minutes on a dual carriageway.

It's mainly town driving though, so my calculations make more sense really.
 
mjt said:
But that's the thing; how do you know you're putting in the same amount of fuel every time? Who's to say that first click is the same amount every fill?
Even using my method of putting in as much as I can to the nearest euro (OCD), the readout must be totally wrong considering it's saying I've used 5 litres less than I have..

Sucks though, this car's not as economical as it's making itself out to be :p
Not surprising though considering the average speed over the last 4 tanks has been 30 mph..
Click to expand...

Try and use the same petrol station twice, or better still, the same pump
 
obc on my old golf (1995) is scarily accurate. tenths out.

on the mk4 (even tho i dont have obc) they are freakishly innacurate. in the region of 10mpg out
 
Trip computer sounds like it is out. As Fox says it doesn't matter how you fill it as long as it is consistent. Personally I go for fill to the click, pull the pump out by about a third then trickle it in to the second click.

The only way you will get any error worth noting is if you fill it flat out until it clicks for the first time then stop as the pumps can stop prematurely if you are sloshing fuel in that quick.

Oh and for comparison (not that there really is one to make) my Octavia over reads by about 2mpg, the Grande Punto seems to be pretty much spot on.
 
Next time you fill up full check the obc for distance to empty. If this is way over what you normally manage then the computers wrong. My car is about 4mpg out, when I fill up it says i have 440 till empty, but have only once ever managed 405.
 
Usel said:
Next time you fill up full check the obc for distance to empty. If this is way over what you normally manage then the computers wrong. My car is about 4mpg out, when I fill up it says i have 440 till empty, but have only once ever managed 405.
Click to expand...

I'm not sure how that's going to work, the distance till empty will be calculated using your current average mpg, as this changes the distance till empty will change.
 
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