huge difference between board computer and real mpg

[gergely]

Hi all, I have a 150PS SWB 4Motion DSG T6 Multivan (Caravelle for you UK folks) - ie heavy as F... The boardcomputer says my fuel consumption is approx 7.8l / 100 km over the last 2-3000km whereas when measured by fill-ups it is more like 10 litres. I don't consider 10 litres as VERY high given that I used it for shorter trips during the winter therefore with the aux heater running quite a lot and when on highway, my speed tends to be 120-125km/h. BUT, the difference between the MFD and real life is huge!
when it comes to AdBlue range I filled up recently and it showed 4500km for a 3-400km and then dropped to 4000. That's okay... But then, probably after a less spirited 500km of drive it went back to 4500km again and it's still there after another 500km. AdBlue consumption, therefore the range, would be proportional to Diesel consumption so either the range calculation is off for AdBlue too or it really doesn't consume that much as much I pay for fuel when filling up (using the same station even the same pump mostly).

Okay so my concerns:
1. does my van really consume 10 litres, does it seem real given the profile?
2. how can it be that I usually fill up 77 litres just shortly after it asks for being filled up (80l fuel tank, obviously) and still says I have 100 km range? This would suggest more like a 90 l tank which doesn't exist.
3. how is the MFD so freakin' off with it's calculation? Is it similar for you guys?

For my imperial friends:
7.8 l / 100 km = 30 mpg
10 l / 100 km = 23 mpg
AdBlue range starts with 2800 mi, dropped to 2500 after driving 250 miles and went back to 2800 and sits there after another 300 miles

 

[mmi]

during the winter therefore with the aux heater running quite a lot

the difference between the MFD and real life is huge!

The fuel consumed by aux heater is not included in the MFD reading.

1. does my van really consume 10 litres, does it seem real given the profile?

For the profile - winter (dense air + Webasto), speed over 120 km/h - I would say normal.

2. how can it be that I usually fill up 77 litres just shortly after it asks for being filled up (80l fuel tank, obviously) and still says I have 100 km range? This would suggest more like a 90 l tank which doesn't exist.

Well, the 80l (or 70l) is just the nominal capacity - the actual capacity is a different story. On the Forum there are posts about brave people having driven well beyond zero range on MFD - and managed to put in more than the nominal volume (just can't find a post now).

when it comes to AdBlue range I filled up recently and it showed 4500km for a 3-400km and then dropped to 4000. That's okay... But then, probably after a less spirited 500km of drive it went back to 4500km again and it's still there after another 500km. AdBlue consumption, therefore the range, would be proportional to Diesel consumption so either the range calculation is off for AdBlue too or it really doesn't consume that much as much I pay for fuel when filling up (using the same station even the same pump mostly).

Remaining range for the AdBlue is just an estimate based on recent consumption/driving style - thus the estimate can indeed climb up. Actually the same can be seen also in fuel remaining range counter.

Then the AdBlue consumption depends on NOx emissions. When driving very economically very little AdBlue is needed. However, blasting 120 km/h is a different story - a lot of AdBlue is injected to neutralize the emissions

Well, some other fascinating MFD & fuel consumption features here:

[T6_measured] Fickle fuel consumption on MFD – encore

In March 2020 finally got the long-awaited ECU (Engine Control Unit) software update. Unfortunately it took a while to get a good chance to do some measurements to try to figure out what has been changed. So finally a few weeks back dusted the VCDS and collected all kind of intriguing data. The...

www.t6forum.com

Other AdBlue features here [EDIT: link corrected]

[T6_measured] NOx emissions, AdBlue consumption, DPF regeneration - Before and after ECU update

Introduction An update campaign for T6 EU6 diesel engines took place in 2019-20. Affected engines were on van of model years MY16-MY18, including some of MY19. For M1 class (passenger carriers, VIN WV2...) the update was an official recall. For N1 class (VIN WV1....) the update was not...

www.t6forum.com

Then to keep eye on AdBlue counter details in the engine:

[T6_measured] Monitoring DPF regeneration, DPF condition, EGR operation

Here's another approach for T6 DIY enthusiasts - taking advantage of very popular Carista OBDII-reader device - combined with an Android app. Ingredients: T6 EU6 engne: CXEB,CXEC,CXFA,CXGA,CXGB,CXGC,CXHA,CXHB. Carista OBDII device (also other generic OBDII (ELM327) devices will work) Android...

www.t6forum.com

 

[Robert]

Well, the 80l (or 70l) is just the nominal capacity - the actual capacity is a different story. On the Forum there are posts about brave people having driven well beyond zero range on MFD - and managed to put in more than the nominal volume (just can't find a post now).

I have a 70l fuel tank and managed to fill up quite a little bit more than 70l 2 days ago.

 

[mmi]

Well, after a fill-up a raw data extract behind the MFD readings.

 

[gergely]

I have a 70l fuel tank and managed to fill up quite a little bit more than 70l 2 days ago.
View attachment 240073

I wouldn't mind the consumption for €0.00 per litre!

 

[gergely]

Thanks for your detailed reply @mmi!

The fuel consumed by aux heater is not included in the MFD reading.

This alone might explain the difference then. I didn't mention that I have tweaked the aux heater to turn on below 8C instead of the factory 5C given the mild winter we have here (so many "chilly" mornings @6C and the heater not helping ).

For the profile - winter (dense air + Webasto), speed over 120 km/h - I would say normal.

It's a very comfy brick, I know but reading about 5-6-7 litres / 100 km all over the forum made me wonder. Having read your analysis in the linked threads about how much fuel the DPF regen takes - and I surely have had some failed ones despite leaving it running when noticing the elevated revs and fans at arrival - helps to understand it too. Not the difference between MFD and real life but the consumption alone.

Well, the 80l (or 70l) is just the nominal capacity - the actual capacity is a different story. On the Forum there are posts about brave people having driven well beyond zero range on MFD - and managed to put in more than the nominal volume (just can't find a post now).

My 77 litres is measured until when the first click comes (pump turns off) and that's definitely not precise. But 80 l size vs 77 l filled up vs 100km range left on MFD vs 10 l / 100km average still hurts my OCD. Humans can do laser surgery and travel to moon but the car can't tell such a basic info. Anyway, 1st world problem.

Remaining range for the AdBlue is just an estimate based on recent consumption/driving style - thus the estimate can indeed climb up. Actually the same can be seen also in fuel remaining range counter.

Then the AdBlue consumption depends on NOx emissions. When driving very economically very little AdBlue is needed. However, blasting 120 km/h is a different story - a lot of AdBlue is injected to neutralize the emissions

And when it comes to estimations, well, I wrote my thoughts already. Good that we can turn off some data being shown on MFD and that's might be what I'm going to do with avg consumption and range. AdBlue remains as it can't be turned off but fill up volume actually makes sense so I don't mind.

I suspect the diesel quality to be a factor in AdBlue consumption too (as it is a factor in NOx emissions). The fuel (both gasoline and diesel) in Hungary doesn't have the highest reputation when it comes to quality. Visiting Croatia and Austria regularly (not with the van yet) and my consumption goes down at least by 0.5 litres. I may try a premium diesel some day.

 

[Pauly]

Looks like you have aftermarket wheels, a larger rolling circumference may upset the onboard computer ie calculated distance vs actual distance travelled ?

Has the van ever been remapped ?

Lots of factors

 

[Murdoch]

The MFD reading is wildly optimistic in my experience

 

[gergely]

Looks like you have aftermarket wheels, a larger rolling circumference may upset the onboard computer ie calculated distance vs actual distance travelled ?

Has the van ever been remapped ?

Lots of factors

Wheels are 225/60 R17, certainly not factory circumference but waaaaay less than how much the MFD is off. Speedo is actually perfectly correct with this wheel beeing just 1-2% bigger in size than 235/55 R17 would be. Btw I bought it like this, next purchase might be factory size but I also like that it's 1cm slimmer might even make consumption better despite the bigger circumference. +1: being larger, it makes the actual travelled distance higher than what the computer thinks ie making optimistic MFD calculations (fairytales) closer to real life measurements, not farther away.

Dunno about being remapped, most probably not.