Mismatch between fuel and display

[GraemeW]

With all my cars I have kept a record of the individual refuelling stops against mileage. The MG3 hybrid is no exception but I am seeing some concerning figures. At the moment, over 1080 miles I have recorded only 43mpg so, last weekend, with a 400mile run up and down the A1 planned. I filled up at the start of the run and at the end. I averaged, from the fuel purchased, 41mpg. If that isn't bad enough, the overall mpg in the dash display (reset at the start) was reading 51.4 (433 miles; average 53mph). I cannot understand why there is such a big difference. Anyone else compared fuel purchases with display values? Anyone know why there is a difference? I have checked and rechecked my maths.

 

[Coulomb]

Are you perhaps calculating using US gallons instead of imperial gallons? US gallons are 4/5 (0.8) of an imperial gallon. I note that 41/51.1 = 0.80. But I can't imagine how you would do that, unless you are converting from litres.

 

[Pigger1972]

With all my cars I have kept a record of the individual refuelling stops against mileage. The MG3 hybrid is no exception but I am seeing some concerning figures. At the moment, over 1080 miles I have recorded only 43mpg so, last weekend, with a 400mile run up and down the A1 planned. I filled up at the start of the run and at the end. I averaged, from the fuel purchased, 41mpg. If that isn't bad enough, the overall mpg in the dash display (reset at the start) was reading 51.4 (433 miles; average 53mph). I cannot understand why there is such a big difference. Anyone else compared fuel purchases with display values? Anyone know why there is a difference? I have checked and rechecked my maths.

You got the heater on?? If that's on the electric motor won't kick in until car really warmed up..also was car in eco mode?? Did you use the cruise control and finally what average speeds did you do??

 

[Coulomb]

You got the heater on??

Surely that's irrelevant here as the car must take that into account. The original post is about reported actual mpg, not anything estimated.

 

[Pigger1972]

Surely that's irrelevant here as the car must take that into account. The original post is about reported actual mpg, not anything estimated.

Not as much as you would think..like I said the electric motor will not kick in until the car is warmed up because the engine needs to be on for that..you get better economy if you don't use the heater much..

 

[Topbloke]

Hi there Graeme, I am going out now and so will get back to you later with full details but I have recorded trip mpg record as against actual fuel purchased/used for that same trip 9 times now and for every occasion trip reports better economy than actual fuel usage which is alarming and most certainly misleading, I have not taken the issue up with dealer as yet

 

[Everest]

Not as much as you would think..like I said the electric motor will not kick in until the car is warmed up because the engine needs to be on for that..you get better economy if you don't use the heater much..

That makes no sense, for the reasons @Coulomb mentioned above

for every occasion trip reports better economy than actual fuel usage which is alarming and most certainly misleading

Maybe MG just employed some redundant VW engineers from 2015, who had learnt the skills of making cars appear more efficient than they are?

 

[GraemeW]

Are you perhaps calculating using US gallons instead of imperial gallons? US gallons are 4/5 (0.8) of an imperial gallon. I note that 41/51.1 = 0.80. But I can't imagine how you would do that, unless you are converting from litres.

I used 0.22 litres to gallons

You got the heater on?? If that's on the electric motor won't kick in until car really warmed up..also was car in eco mode?? Did you use the cruise control and finally what average speeds did you do??

The heater was set to 21 but the point is the incar display (the one everyone takes as true) showed 52 for the full length of the trip. This is "not bad" (would have liked better) indicating some contribution from the hybrid elements. But it was costing much more in petrol!

Hi there Graeme, I am going out now and so will get back to you later with full details but I have recorded trip mpg record as against actual fuel purchased/used for that same trip 9 times now and for every occasion trip reports better economy than actual fuel usage which is alarming and most certainly misleading, I have not taken the issue up with dealer as yet

Look forward to seeing your input, Topbloke

Not as much as you would think..like I said the electric motor will not kick in until the car is warmed up because the engine needs to be on for that..you get better economy if you don't use the heater much..

With respect but you are missing the point: x gallons of fuel for y miles. That is the only valid calculation for mpg and it's a lot worse than the car display is telling me.

 

[Topbloke]

I used 0.22 litres to gallons


The heater was set to 21 but the point is the incar display (the one everyone takes as true) showed 52 for the full length of the trip. This is "not bad" (would have liked better) indicating some contribution from the hybrid elements. But it was costing much more in petrol!


Look forward to seeing your input, Topbloke


With respect but you are missing the point: x gallons of fuel for y miles. That is the only valid calculation for mpg and it's a lot worse than the car display is telling me.

OK Graeme and so I filled up again when out earlier today and so data for 10 different trips on my MG trophy EV hybrid was as follows:
trip showed 46.3 actual MPG was 44.83 difference being 1.87
49.6 45.43 4.17
46.3 43.83 2.47
47.1 43.41 3.69
47.9 44.71 3.19
51.4 48.48 2.92
56.5 53.07 3.43
55.7 53.62 2.08
46.3 41.07 5.23
50.4 46.94 3.46

thats all I can tell you but as you can see trip data is certainly not accurate and consistently understating what was actually achieved. Maybe some other subscribers might monitor their vehicles so see if they are more accurate & true than my experience to date ?

 

[Everest]

Just thinking outside the box here... do plug-in-hybrids show petrol consumption as well as miles per kWh on their dash? If so, presumably they must somehow ignore distance powered by electric motor as that would have come from externally charged power.

If that is the case (and just guessing here as I've never owned a hybrid)... I'm wondering if the same algorithms have been used in the MG3 with the engineers (conveniently) forgetting that the MG3 is not a plug-in hybrid and all power will ultimately come from petrol? In other words are there some lines of code that add electric driven miles to the range for the purposes of calculating consumption?

 

[GraemeW]

OK Graeme and so I filled up again when out earlier today and so data for 10 different trips on my MG trophy EV hybrid was as follows:
trip showed 46.3 actual MPG was 44.83 difference being 1.87
49.6 45.43 4.17
46.3 43.83 2.47
47.1 43.41 3.69
47.9 44.71 3.19
51.4 48.48 2.92
56.5 53.07 3.43
55.7 53.62 2.08
46.3 41.07 5.23
50.4 46.94 3.46

thats all I can tell you but as you can see trip data is certainly not accurate and consistently understating what was actually achieved. Maybe some other subscribers might monitor their vehicles so see if they are more accurate & true than my experience to date ?

I guess the in car display figures are the higher ones? The percentage errors you are showing are probably not unusual for most cars. My calcs on previous cars showed similar variations (spreadsheets!).
I spoke to the garage but not particularly interest other than asked me to do another monitored run. Due to do the same 400 mile run in a few weeks so it can wait to then.

MG3 is not a plug-in hybrid and all power will ultimately come from petrol

There are a lot of people who dont seem to realise that fundamental point!

The point you make about calc error I guess is possible although on the long run there was very little EV running.
Thought: Energy used to heat the interior or the seats comes from the icu and will contribute to fuel consumption BUT will not add its share to the miles. That is "wasted" energy, as is heat lost through the rad. I can't think that would be very much though. Does the car measure actual fuel flow or work off the power figures or similar?

SUPPOSING the car was working on US gallons! That would make the 400 mile long run out at 49 mpg whereas the display shows 51.4. Thats close

 

[Everest]

Does the car measure actual fuel flow

I thought they measure fuel flow by knowing injector flow rate + rpm + opening times of injector. Then combine that with either speed for instantaneous mpg display or distance for average consumption since last reset. So should be pretty accurate - my BMW (diesel... am I allowed to say that word on here?) seems very accurate over 1000's of miles.

 

[Pigger1972]

Sorry no crayons to draw a picture...

That makes no sense, for the reasons @Coulomb mentioned above


Maybe MG just employed some redundant VW engineers from 2015, who had learnt the skills of making cars appear more efficient than they are?

 

[KeithS]

With all my cars I have kept a record of the individual refuelling stops against mileage. The MG3 hybrid is no exception but I am seeing some concerning figures. At the moment, over 1080 miles I have recorded only 43mpg so, last weekend, with a 400mile run up and down the A1 planned. I filled up at the start of the run and at the end. I averaged, from the fuel purchased, 41mpg. If that isn't bad enough, the overall mpg in the dash display (reset at the start) was reading 51.4 (433 miles; average 53mph). I cannot understand why there is such a big difference. Anyone else compared fuel purchases with display values? Anyone know why there is a difference? I have checked and rechecked my maths.

I do the same and find the usual discrepancy that has been mentioned above - about 4 mpg less than the trip computer shows.
However when I filled up this week, the difference was markedly worse. The trip showed around 40 mpg, and when calculated it only averaged just under 33mpg, which doesn't make much sense given that would be unusually lower than on any other tank of fuel.

 

[Coulomb]

Does the car measure actual fuel flow or work off the power figures or similar?

I would think that the car has to be measuring fuel flow. Anything else would presumably be hopelessly inaccurate, given how internal combustion engines vary in efficiency with so many variables (air temperature, engine speed, fuel composition, load, etc).

A sudden change in actual MPG could be due to the difference in fuel formula from one fill to the next. Fuels change from summer to winter and from brand to brand. Some fuels have a higher octane rating, and so on.

 

[Topbloke]

I guess the in car display figures are the higher ones? The percentage errors you are showing are probably not unusual for most cars. My calcs on previous cars showed similar variations (spreadsheets!).
I spoke to the garage but not particularly interest other than asked me to do another monitored run. Due to do the same 400 mile run in a few weeks so it can wait to then.



There are a lot of people who dont seem to realise that fundamental point!

The point you make about calc error I guess is possible although on the long run there was very little EV running.
Thought: Energy used to heat the interior or the seats comes from the icu and will contribute to fuel consumption BUT will not add its share to the miles. That is "wasted" energy, as is heat lost through the rad. I can't think that would be very much though. Does the car measure actual fuel flow or work off the power figures or similar?

SUPPOSING the car was working on US gallons! That would make the 400 mile long run out at 49 mpg whereas the display shows 51.4. Thats close

Hi again Graeme just to confirm yes car display figures are higher ones but the differences I quote are not percentages but in MPG and so in real terms % would be much higher almost double on some of these recorded trips. I shall continue to record all future trips and come warmer weather hopefully diffs will become much less although many of these trips were in september onwards, be interested to know what your own future data shows.

 

[GraemeW]

Hi again Graeme just to confirm yes car display figures are higher ones but the differences I quote are not percentages but in MPG and so in real terms % would be much higher almost double on some of these recorded trips. I shall continue to record all future trips and come warmer weather hopefully diffs will become much less although many of these trips were in september onwards, be interested to know what your own future data shows.

Thank you Topbloke (you live up to your name!). I will continue to monitor carefully and see how it goes