MFD MPG Lies

[ShooglyStu]

I've been monitoring my fuel consumption over the last 2200 miles on my 2016 T6 TDI 102.
The long term average 2 on the MFD gives a figure of around 47mpg which would be brilliant if it were true, but it's a total lie!
Manually calculating the consumption between successive full tanks from the amount of fuel required to brim full gives a more believable figure of 37mpg

The difference is massive!
Why?
Anyone else had such a huge difference?

The MFD estimates fuel flow from the time period that each injector is commanded to open by the engine ECU and the measured fuel pressure, so could there be a problem with the injectors, HPFP or fuel pressure sensor?

 

[DaveD]

Many of us are using non standard wheel / tyre set ups which will affect the MFD reading.
DPF regens will also affect the real life MPG that you are getting.
In any event I wouldn't assume that there is a problem with the injection system, just enjoy your van.

 

[dubber36]

I initially thought bigger wheels but they affect the speedo therefore distance travelled. Both the MFD and pen and paper calculations are using the same recorded distance travelled numbers, so the varient is the amount of fuel used.

If it's been remapped it uses more fuel than the ECU tells it too, so that will affect the MFD figures.

 

[Crazymind]

I find the MFD quite spot on. 29.4 Mpg long term shown, 29mpg real consumption.

 

[The Bear78]

I think most people find the MFD figures to be quite unreliable. The only useful aspect is the real time figure which allows you to adapt your current driving style/gear choice to be the most efficient but the actual number shown is irrelevant.

A couple of years ago I tried to drag out a tank of fuel for as long as possible, in the interest of science. I was commuting about 38 miles each way on fast A roads at the time so could average 50 - 60 mph for most of the journey. I got to 725 miles with 15 miles remaining before I bottled it (Euro 5 LWB 140 6 speed).
On the MFD, the trip figures would always be ~49mpg on the way to work and ~51 on the way home. I don't recall what the long term figure was.
When I filled up it was about 77 litres (80 litre tank obviously) which works out to be 43.7 mpg for 740 miles. Far from the 50 that I was averaging.
I don't have the patience to drive that way anymore (ever again) so don't suppose I'm even in the mid 30's these days!!

 

[The Bear78]

I find the MFD quite spot on. 29.4 Mpg long term shown, 29mpg real consumption.

29 sounds more realistic!

 

[mmi]

The difference is massive!
Why?
Anyone else had such a huge difference?

If the van has Webasto auxiliary coolant heater - the fuel used by it is not included in the MFD calculations.

 

[ShooglyStu]

The van has not been remapped to my knowledge and it certainly doesn't feel like it!
It doesn't have a diesel heater either.

Over the weekend I did 300 miles on dual carriageway at 50-65 mph plus 150 miles on minor roads and the manually calculated mpg increased to 42.8mpg
The MFD was reset at the start and by the end was reading 47mpg again.
So, a smaller difference but still pretty big.

 

[jamesmack]

Will check if mine is out when I do my next full tank, since having mine about a week and a half ago.. I've done about 500 miles and says I've averaged 31mpg over that time mixed driving

 

[Ayjay]

I've got to say that whenever somebody pops up to tell us that their van returns 50-60MPG (or whatever) my first thought is always to wonder whether they just look at the MFD or do an actual measurement!

 

[Bargy62]

I've got to say that whenever somebody pops up to tell us that their van returns 50-60MPG (or whatever) my first thought is always to wonder whether they just look at the MFD or do an actual measurement!

or telling porkies

 

[Alster]

Haven’t checked recently but when I first had my van the mfd was pretty accurate. I get around 32 for work (shorter journeys) and around 40-42 on a long run. (T32 199 Highline Kombi)

 

[Huba]

My MFD is currently saying the average is 48.2mpg, this is on a MFD trip 2 of ~1500miles. That suggests 740 miles out of a 70l tank. I reset the odometer trip every time I fill up and just checked have done 255 miles with a range of 455 miles (2/3 of a tank left). That's pretty normal for me on my commute, winter / spring / autumn. Has always been similar figures. I have a 2018 102 5 speed. Tyres are van contact eco (50psi), which are rock hard ride but give better mpg (1-2) than the van contacts that were on before (even harder)

I'm interested to know if it is accurate as in the summer on my commute driving carefully regularly get just over 50 mpg. I usually fill up with range of about 170-200 and the trip odometer is usually near 500. I will calculate it more precisely to check when filling up, reading the above posts it suggest my figures are optimistic

When we're away in the van it drops to low 40s. Anything over 50mph and mpg drops rapidly in my experience

 

[dubber36]

I'd say that mine was consistently 10% optimistic. The long term average was said to be 38mpg, when in reality it was more like 34mpg. Filling the tank (70 litres) would mostly show a range of 600miles, but in reality I'd be lucky if I got 500miles.

 

[ShooglyStu]

I'm quite happy to get 40-ish mpg out of a 2 tonne brick, it's only the discrepancy with the MFD that's bothering me.

It could be an issue with the high pressure fuel sensor or an internal fuel leak somewhere (injectors maybe?), there's no fuel spillage under the van, it starts immediately and there are no fault codes.

How much fuel is used for DPF regen and does this get accounted for by the MFD display?

I'm going to run a few tanks of Redex through it to see if that makes any difference.

 

[Crazymind]

You made a good point there! Forgot to mention recently my van was is for a supposed faulty high pressure pump sensor. They were to replace the pump as well. However recalibration of the sensor fixed the issue and I have to say after that I have noticed the fuel consumption is better than it was before, especially on a short run. 18.5mpg before and now it’s on 22mpg

 

[ShooglyStu]

You made a good point there! Forgot to mention recently my van was is for a supposed faulty high pressure pump sensor. They were to replace the pump as well. However recalibration of the sensor fixed the issue and I have to say after that I have noticed the fuel consumption is better than it was before, especially on a short run. 18.5mpg before and now it’s on 22mpg

Did you have fault codes for the high pressure pump sensor?

 

[bubbadubba]

I average between 400 to 460 miles on a full tank (fill up and then repeat when fuel light comes on) in my 2018 150 manual. My old 140 t5.1 was much much better on fuel. I am sure something's not quite right and it's doing more regens than it should be. I read on here than some people are getting up to 700 miles on a tank.....that's amazing for them but I can't get anywhere near that. I do carry alot of weight in it as its a work van but no different than the old van.......& I drive like an old lady I have been told.
It drives ok as far as I can tell??
What should I be looking at to see if I can cure it's thirsty habit???

 

[Crazymind]

No. It was a weird one. ETC light was on and after the light is on the van goes into limp mode.
They have traced it back to high pressure pump and discovered the sensor was at fault. However they have installed a new software and the light not coming back on yet.

 

[Crazymind]

I average between 400 to 460 miles on a full tank (fill up and then repeat when fuel light comes on) in my 2018 150 manual. My old 140 t5.1 was much much better on fuel. I am sure something's not quite right and it's doing more regens than it should be. I read on here than some people are getting up to 700 miles on a tank.....that's amazing for them but I can't get anywhere near that. I do carry alot of weight in it as its a work van but no different than the old van.......& I drive like an old lady I have been told.
It drives ok as far as I can tell??
What should I be looking at to see if I can cure it's thirsty habit???

After reading the incredible miles people seem to get off one tank of Diesel I have tried once they hyper mile challenge. If you are brave enough to hit 55-60mph all the way to your destination… that’s how you get 700 on a tank…
However if you manage 65mph, 35-38mpg are possible.
In my experience with T5, T6, T6.1 going between 73-78mph always been around 28/30mpg.