Hi all,

 

The MFD indicated mpg in my 2005 Octavia has been consistantly about 25% for the two and half years I've owned it. I've always assumed this was just due to a poorly set VAG-COM calibration factor and not worried about it. Someone has just just suggested to me that it might be due to a faultly sensor, are they likely to be right? The car is running fine as far as I can tell.

 

Thanks for any input .

Edited 7 September, 201312 yr by Ultrasonic

It's possible. A sensor that badly out of spec should have stored a code, so a VCDS scan would be useful exercise.

 

Having said that, it's likely the calibration factor is just out.....although I've never heard of a 25% error.....5-15% is more typical. This can be corrected with VCDS. 

Thanks.

 

In the time I've had the car it's had both ABS and parking sensor faults fixed at a main dealer. Would it be reasonable to assume that at these times they'd have checked for fault codes and picked up any significant sensor faults? I'm tempted to think they wouldn't have ignored any faults as they could have made money out of fixing them.

 

Although I guess I should probably splash out on a OBD2 lead to be sure...

 

Edit: I should perhaps add that the distances reported by the MFD are bang on compared to my SatNav and Google maps, so it's not an error in this that is causing the stupidly large error I have.

Edited 7 September, 201312 yr by Ultrasonic

Thanks.

 

In the time I've had the car it's had both ABS and parking sensor faults fixed at a main dealer. Would it be reasonable to assume that at these times they'd have checked for fault codes and picked up any significant sensor faults? I'm tempted to think they wouldn't have ignored any faults as they could have made money out of fixing them.

 

Yes, that's true.

 

Probably just a calibration issue. VCDS can correct it.  

I might get round to correcting it at some point, but I've kind of got used to it . I know what it means, even if no one else would!

Hi all,

 

The MFD indicated mpg in my 2005 Octavia has been consistantly about 25% for the two and half years I've owned it. I've always assumed this was just due to a poorly set VAG-COM calibration factor and not worried about it. Someone has just just suggested to me that it might be due to a faultly sensor, are they likely to be right? The car is running fine as far as I can tell.

 

Thanks for any input .

There is no sensor as such.  The ECU keeps a tally of how much fuel it has asked the injectors to fire and combines that with mileage covered to give mpg.

 

It could well be a badly calibrated unit but here are some other random thoughts:

 

Over my period of ownership my 2006 1.9TDi gave an average of 53.9mpg on the MFD and 50.8mpg from brim to brim calculations.  What are the 2 figures you get?

 

If the MFD is 25% optimistic then you may have a fuel leak - the MFD only tallies the fuel going to the injectors.

 

Are you sure you are calculating correctly?  Using the litres to imperial gallon conversion of 4.546 litres per gallon and not the US conversion of 3.785litres per gallon.  Beware of calculators with inbuilt convertors or web based convertors and record sites that use US gallons not imperial gallons.

 

Does the MFD give a reasonable estimate of miles left in the tank?  If you get vastly more or vastly less miles per tank than the MFD suggests then it is the calibration that is out.

Are you sure you are calculating correctly?  Using the litres to imperial gallon conversion of 4.546 litres per gallon and not the US conversion of 3.785litres per gallon.  Beware of calculators with inbuilt convertors or web based convertors and record sites that use US gallons not imperial gallons.

 

Very good point. Inadvertently using an online calculator that uses US gallons would introduce a 20% error to the calculations.  

Sorry, Ive been away for a couple of days and only just seen your reply eccleshill, thanks for your thoughts.

 

There is no sensor as such.  The ECU keeps a tally of how much fuel it has asked the injectors to fire and combines that with mileage covered to give mpg.

 

Is perhaps what is used the time the ECU tells the injectors to be open for? This would then mean the true fuel usage would depend on how accurately the injectors operate for the requested time, the fuel pressure and the nozzle sizes (and probably other things I can't think of quickly). Are there definitely no sensors involved, to measure fuel pressure for example?

 

It could well be a badly calibrated unit but here are some other random thoughts:

 

Over my period of ownership my 2006 1.9TDi gave an average of 53.9mpg on the MFD and 50.8mpg from brim to brim calculations.  What are the 2 figures you get?

 

I keep a very detailed spreadsheet, and do have indicated and brim-to-brim calculated mpg data for 56 refuels, but I've never bothered to calculate the (properly weighted) average indicated mpg. The average brim-to-brim mpg is currently 56.3 mpg (over about 2.5 years and 23,000 miles). My ~25% overestimate came from a linear best fit through a graph of all of the indicated and measured data, although actually I now realise 19% is closer to the mark (I just inverted the gradient to get to 25% but neglected the offset term). To give you an idea though, my most accurate recent data had an indicated 70.0 mpg compared to a measued 59.0 mpg (I say most accurate as this was my last refill from close to empty, that was also deliberately carried out using the same fuel pump as the previous fill).

 

Although I've tracked mpg with my own spreadsheets for years I recently put the data for my current car into Fuelly.com so I could share it on another forum. So I may as well bore you with the data too ;-)

 

 

I do work at driving economically by the way. My best of 65.9 mpg was achieved for mostly dual carriageway driving at an indicated 60 mph (~58 mph true speed) in very hot summer weather.

 

If the MFD is 25% optimistic then you may have a fuel leak - the MFD only tallies the fuel going to the injectors.

 

I doubt I have a fuel leak as otherwise my fuel economy would likely to lower than it is? The mpg figures I get appear typical from what I've seen online, including Honest John real mpg data.

 

Are you sure you are calculating correctly?  Using the litres to imperial gallon conversion of 4.546 litres per gallon and not the US conversion of 3.785litres per gallon.  Beware of calculators with inbuilt convertors or web based convertors and record sites that use US gallons not imperial gallons.

 

That's a sensible question, but yes, I'm certain I'm calculating the true mpg correctly.

 

Does the MFD give a reasonable estimate of miles left in the tank?  If you get vastly more or vastly less miles per tank than the MFD suggests then it is the calibration that is out.

 

The miles left per tank is very optimistic. I've never bothered to check if this exactly tallies with the MFD mpg, but I assume it does.

Edited 10 September, 201312 yr by Ultrasonic

LOL!!  Thought I was the only anal fuel consumption analyser!  Some more random thoughts and some comparison data:

 

I too plot brim-to-brim vs trip computer mpg.  For my 2006 1.9TDi over 36k miles and 77 fill-ups the linear fit is

 

brim-to-brim = trip-computer x 0.9457 with an r squared of 0.78

 

NB no off-set.  The linear trendline MUST go through the zero/zero origin.  Think about it.  Fill your tank, sit statioary with the engine running on tickover for 24hrs and refill the tank to determine how much has been used.  You have travelled zero miles and have used (say) 20 litres = zero mpg brim to brim.  Your trip computer will also say zero mpg.

 

My Skoda trip computers have been pretty consistent in being around 5% optimistic:

For my 2010 1.8TSi over 29k and 90 fill-ups the linear fit is brim-to-brim = trip-computer x 0.9529 with an incredible r squared of 0.90

For my current 2013 1.6TDi over 3,900 miles and 7 fill-ups the linear fit is brim-to-brim = trip-computer x 0.9545 with an r squared of 0.4 (small no of data points)

 

Your brim-to-brim is a reasonable looking number, the trip computer is a little too high to be believed!  So it looks like your trip computer calibration is out!  I understand that there is a number that can be tweaked via VCDS to bring them into line. 

 

Just looked at your fuelly.  There are an awful lot of very small fill-ups.  I reckon that when you fill up then even using the same pump your fill-to-fill precision is 2 to 3 litres.  That means a potential error on a 15litre fill up of 20%.  Though to be fair your individual fuelly mpg variation doesn't suggest such a wide error and over a large number of fills the error on the overall average tends to diminish

 

Are those fuelly figures really brim-to-brim?  Have you really been driving a handful of miles and re-brimming with 9litres?  Think of the fuel you are wasting with multiple filling station stops and permanently carrying a full tank. 

LOL!!  Thought I was the only anal fuel consumption analyser!  Some more random thoughts and some comparison data:

 

I too plot brim-to-brim vs trip computer mpg.  For my 2006 1.9TDi over 36k miles and 77 fill-ups the linear fit is

 

brim-to-brim = trip-computer x 0.9457 with an r squared of 0.78

 

As you're probably the only person who may ever have the slightest interest in my graph (!), here you go:

 

 

 

NB no off-set.  The linear trendline MUST go through the zero/zero origin.  Think about it.  Fill your tank, sit statioary with the engine running on tickover for 24hrs and refill the tank to determine how much has been used.  You have travelled zero miles and have used (say) 20 litres = zero mpg brim to brim.  Your trip computer will also say zero mpg.

 

I understand what you're saying but do actually still think it's best not to force a linear fit through zero. The way my car calculates mpg is clearly screwy, so I wasn't sure whether the response would be linear, or indeed show 0 mpg if I leave the car idling. I therefore consciously decided not to force the fit through zero. Of course this distinction makes no real difference to me, let alone anyone else!

 

My Skoda trip computers have been pretty consistent in being around 5% optimistic:

For my 2010 1.8TSi over 29k and 90 fill-ups the linear fit is brim-to-brim = trip-computer x 0.9529 with an incredible r squared of 0.90

For my current 2013 1.6TDi over 3,900 miles and 7 fill-ups the linear fit is brim-to-brim = trip-computer x 0.9545 with an r squared of 0.4 (small no of data points)

 

It's interesting how consistant that is across multiple models. I'm tempted to think Skoda are aiming for it to read ~5% high. I think it would make sense for a manufacturer to make sure it wasn't likely to read low...

 

Just looked at your fuelly.  There are an awful lot of very small fill-ups.  I reckon that when you fill up then even using the same pump your fill-to-fill precision is 2 to 3 litres.  That means a potential error on a 15litre fill up of 20%.  Though to be fair your individual fuelly mpg variation doesn't suggest such a wide error and over a large number of fills the error on the overall average tends to diminish.

 

I used to think comparatively small fill ups would be wildly innacurate as well, but as you see my fuelly data suggests otherwise. I also now have a ScangaugeII, which I've calibrated to be accurate to within 1% of my brim-to-brim results, even for short fill-ups. There is also not a linear conversion factor between my SGII mpg figures and those from my MFD (or even a one to one relationship). I had assumed that the SGII was using a better algorithm to convert data from various sensors to calculate mpg. If as you suggest there really are no sensors involved (are you certain?) I'm now rather at a loss why this might be.

 

Are those fuelly figures really brim-to-brim?  Have you really been driving a handful of miles and re-brimming with 9litres?  Think of the fuel you are wasting with multiple filling station stops and permanently carrying a full tank.

 

The few recent short fill ups were deliberate whilst I was trying to investigate driving style effects on mpg during hopefully reasonably consistent weather conditions. The fuel saved by running with a virtually empty tank compared to a full tank is pretty small anyway. Here's a rough calculation:

 

If I only ever filled my tank up to half-full I'd save 23.1 kg of weight (27.5L @ ~840 g/L). Going by data from this interesting report I calculate this would result in an improvement in mpg of ~0.65% for their 'combined' driving cycle, and rather less than that for steady state driving. So that would be a saving of 5.25 L of fuel per 10,000 miles at my current 56 mpg average. So I certainly don't worry about a few short fillups. I do aim to run my tank down till the warning light comes on , but what typically happens is that I then have a 360 mile round trip to visit other family members at a weekend, and I'll fill up before I leave so I don't have to worry about buying fuel when away.

 

Edit: the other reason for some short fill ups recently was I was trying to separately assess mpg for my work commute (just 9 miles each way, part town driving) and longer trips.

Edited 11 September, 201312 yr by Ultrasonic

:rofl:  What a pair of geeks eh?  Hope nobody else is watching...

 

That graph with an R squared of 0.93 tells me the calibration on the mfd is definitely the culprit.

 

Here is the thread on how to recalibrate

http://www.briskoda.net/forums/topic/123966-fuel-computer-calibration/?hl=computer%20calibration

Thanks for the link. Although the +/- 15% range won't be enough for me if it is currently set at the supposedly standard 100%. Hopefully mine is currently set lower.

 

I may get round to looking at this some time, but as I've said it appears that my SGII will be more accurate even after a VCDS adjustment. I bought the SGII to be able to see load information, with a view for getting a feel for this and then selling it on, but I've got used to it now and may hand on to it. Having a more meaningful engine temperature display, and the possibility to monitor boost are handy too.