MFD MPG Lies

[Crazymind]

Following this thread I have kept an eye on fuel consumption and MFD. This week I have driven from Glasgow to Laval (France) and back.
Outside Temperature today from London was around 18celsius and for the first time in one Year the consumption at my usual speed (Acc set to 73) was 38 to the gallon! Roadworks and M1, lots of stop and go and deviation to A66 due to an accident. It took an awful 12 hours to drive back home… 56mph average speed and 37.6mpg brim to brim.

 

[ShooglyStu]

410 miles, 50.93 ltr to fill = 36.6 mpg

MFD Ave(2) says 46.4 mpg

 

[mmi]

410 miles, 50.93 ltr to fill = 36.6 mpg

MFD Ave(2) says 46.4 mpg

Fascinating - by doing a bit of number manipulation I get that the MFD is -26.8% on the optimistic side on fuel consumption. Thus to get actual fuel consumption the MFD numbers need to be divided by (1+0.268).

---

By doing the same math on my 164 fill-ups this far shows mine is in average -3.0% off (on optimistic side). As the fill-ups are not always perfectly the same - not exactly level, some foaming, etc - there is some variation when MFD Ave(2) is compared with the actual filled amount. Below the variations plotted.



How to read: e.g. the bar at -2.0% (on x-axis) shows that there were 56 fill-ups where the "error" was between -4.0% and -2.0%. Thus all values very near the average -3.0% are shown on this bar.

Noted twice deviating more than -8% off (the leftmost bar),

Spoiler

the "errors" were actually -11.6% and -9.8% - possibly underfilled at previous fill-ups because those were on the positive side

Also twice deviating more than 2% but less than 4% (the rightmost bar) - possibly underfills

So at least in my case the MFD Ave(2) seems to be fairly precise - though not very accurate

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I guess you need to verify that the MFD hasn't been tweaked... below the factory setting found in control module 17-Instruments.

IDE00499-Display correction of consumptions and operating range

100 %​

 

[gergely]

Just posted this before noticing you have a very similar thread.

huge difference between board computer and real mpg

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...

www.t6forum.com

Interesting how similar your observations are...

 

[ShooglyStu]

I don't believe that Bosch or Siemens or whoever supplied the software that calculates fuel consumption would deliberately falsify the displayed value, I think it's more a case of garbage input = garbage output. So the question is - what input is garbage and why is this not detected?

 

[DXX]

I would think it’s a simple calculation based on fuel temperature, fuel volume and odometer.
The obvious error being the odometer with different wheels / tyres / tyre pressures.

 

[ShooglyStu]

I would think it’s a simple calculation based on fuel temperature, fuel volume and odometer.
The obvious error being the odometer with different wheels / tyres / tyre pressures.

I have compared the odometer with GPS tracks and the difference is less than 2%

The fuel volume will be calculated from the duration of each injection event, the common rail fuel pressure and temperature, so possible factors could be sticky, clogged or leaky injectors or faulty fuel pressure or temperature sensors, but no fault codes are logged.

Then there is a complicated averaging algorithm going on. For example, the extra fuel used for DPF regeneration is averaged over a longer period than the regeneration event itself, to avoid wild fluctuations in displayed mpg, so another potential source of error.

 

[roadtripper]

In the interests of science I was able to do a long run at 50mph overnight the other week where I didn't get in the way of the heavies too badly.

Over 443 miles the van indicated 46.1 MPG (it went over 50 on the motorway leg) and I filled up 42.75litres of Shell V Power.

I make that 47.1 mpg calculated which isn't that off.

I'm running factory wheels and suspension.

 

[DaveD]

In the interests of science I was able to do a long run at 50mph overnight the other week where I didn't get in the way of the heavies too badly.

Over 443 miles the van indicated 46.1 MPG (it went over 50 on the motorway leg) and I filled up 42.75litres of Shell V Power.

I make that 47.1 mpg calculated which isn't that off.

I'm running factory wheels and suspension.

Jeez, my resolve would have failed after 3 minutes.
No, being honest, 2 minutes.
I bet you can put a fruit pastille in your mouth and not chew it.

 

[gergely]

I don't believe that Bosch or Siemens or whoever supplied the software that calculates fuel consumption would deliberately falsify the displayed value, I think it's more a case of garbage input = garbage output. So the question is - what input is garbage and why is this not detected?

I don't think they falsify, I believe they're designed poorly - which would come by no surprise - not taking into account some things. As @mmi pointed out in my thread, one of these factors are the aux heater consumption, that one alone plays a huge role in my false readings. This is not falsification this is poor design (or a feature, it depends who you ask).
Another factor might be wheel size but that doesn't make readings 20% off.
I have no codes just as you.

 

[roadtripper]

Jeez, my resolve would have failed after 3 minutes.
No, being honest, 2 minutes.
I bet you can put a fruit pastille in your mouth and not chew it.

Ha, the endless consumption of Imperial Mints stashed on the driver's snack shelf (I assume that's what everyone uses the top door shelf for?) says otherwise.

In this case it was simply my distant Yorkshire heritage at play, if I arrived too early I'd need more than the 3 free hours at Tebay to fit in with the rest of the itinerary and was worried they might charge the higher motorhome rate (after all I was napping in what looks like one)

 

[roadtripper]

In the interests of science I was able to do a long run at 50mph overnight the other week where I didn't get in the way of the heavies too badly.

Over 443 miles the van indicated 46.1 MPG (it went over 50 on the motorway leg) and I filled up 42.75litres of Shell V Power.

I make that 47.1 mpg calculated which isn't that off.

I'm running factory wheels and suspension.

So update on this under less controlled conditions we're roadtripping to the Brecon arena this weekend.

Now I'm more sure I'm keeping the Caravelle long term I've decided to switch to full Shell V-Power for a bit.

I've always been sceptical of the improved economy claims but this weekend driving at my more normal 60ish 4 up with a weekend for of gear I easily made an indicated 48 mpg - which is 10 mpg more than I do on normal Shell + Millers

Maybe it's a fluke, maybe there's something in it. Will keep an eye on it and maybe do some more controlled runs. Possibly the remarkable mileage I got from the long overnight run was due to fuel choice more than driving style?

 

[Drive Wayne]

Is it just my local Marks and Expensive BP station that seemingly gives me no fuel for the first few seconds of filling up? The money counter is counting but no fuel seems to flow for the first few seconds. Am I being overly skeptical? Is this a 'thing'? Am I indeed of Yorkshire heritage?!

 

[roadtripper]

This is the whole "minimum delivery" thing. People assume it's minimum payment but that's not the case, it's the minimum amount you need to fill up for the measurements to be in tolerance to account for things like startup.

If the minimum delivery is 2 litres you can happily buy and pay for 1 litres - you just can't complain to trading standards if it turns out to only be 0.8 litres.

 

[Lubrown]

It’s not just the mpg that seems fake news from the dash - water temperature is too according to Carista live data.
Tried both the van and golf at different stages of engine warmth. The van was 105 deg according to carista while the needle sat at its uncannily steady favourite of 90 and the golf got to 76 deg from cold before I switched it off as I’d finished playing yet the needle quickly rose to 90 again and stayed steady. It appears 90 is there as an arbitrary figure unless the water temp is at least 15 either side. Either that or the temp gauge in the dash uses a different sensor to the one Carista uses for the water (I can’t see that being the case?).

 

[Lubrown]

This is the whole "minimum delivery" thing. People assume it's minimum payment but that's not the case, it's the minimum amount you need to fill up for the measurements to be in tolerance to account for things like startup.

If the minimum delivery is 2 litres you can happily buy and pay for 1 litres - you just can't complain to trading standards if it turns out to only be 0.8 litres.

Yes, Auto Shenanigans (chap on YouTube) did a video on this. He drove round testing the theory at different filling stations too.
He also did one on ethanol content.
And has many videos on motorways and their oddities. It’s his dry sense of humour that makes them watchable.

 

[roadtripper]

I thought it was a well known fact that the temperature was "smoothed" over 90 so that Joe Public just gets used to the normal operating range being "90"

If I want a clear picture I switch to lil temperature which isn't.

 

[roadtripper]

Yes, Auto Shenanigans (chap on YouTube) did a video on this. He drove round testing the theory at different filling stations too.
He also did one on ethanol content.
And has many videos on motorways and their oddities. It’s his dry sense of humour that makes them watchable.

There's a button specifically for that

 

[Huba]

So this morning I filled up and worked out the mpg on mine and it was 41.6mpg (406 miles) vs 48.5 on the MFD (1600 miles) so at least on mine I can confirm the MFD is optimistic. Would say I've had regens in the 406 miles, a few short runs and a compressor fridge running occasionally (so charging the leisure battery), so it may be a bit lower than it could be

I'll continue to monitor, but suggests the 50 mpg I'm getting in the summer is likely more like 43-44 mpg

So as an update to the above (T32 102 SWB) figures, I was waiting till I got some better figures in the warmer weather. Filled up this morning:

MFD - 53.3
Actual - 44.83
Distance - 482
Usage - Commuting, 1 No. regen, charging leisure battery occasionally but not to often because solar has been keeping it fullish

So I'm about 8 mpg lower than the mfd. Which is quite a lot. I'm resetting the trip 2 every refill now.

I have a couple of times got 60mpg on my 25 mile commute (hot day, no hold ups) so over 50 actual is possible in absolute ideal conditions

Also drive a mk2 Fabia petrol 1.2 and that does 53-54 no problem and requires much less caressing to get to those figures than the van

 

[Crazymind]

You could probably fit the Fabia in the boot of your Transporter.
I think anything around 30MPG is a miracle. Size, weight, and little displacement for such heavy beast…