My Octavia Mk1 130bhp diesel recorded the mpg accurately to within 1mpg.

 

My 150bhp Mk3 diesel seems to err on the optimistic side by 5mpg, or worse.

 

I really don't understand why, though: it knows how much fuel it's burning, and it knows how many miles it's covered.

 

So what's the problem exactly, and can it be fixed?

 

Any thoughts welcome!

Edited 8 October, 20187 yr by Sangrail

There's a conversion factor in the engine logic,should be adjustable via VCDS,mabey even OBDeleven

Is that something the mechanics can do when they service it, for example? And would they recalibrate it, or simply tweak it according to my calculations based on full-tank to full-tank mileage?

i wouldnt trust it even if it was calibrated. Used Fuelly or other similar app to get the most accurate mpg readings

In my experience the accuracy of the mpg trip display varies from individual car to car, you cannot say that one manufacturer or model is consistently more accurate than another and it is more luck than judgment as to what accuracy you are blessed/cursed with.

The individual vehicle's inaccuracy tends to be consistent across similar journey types so once you have ascertained what it is by comparison with actual consumption (monitoring distance covered and refuelling tank to 'full' status for a few tanks) then it is still a reasonably useful comparitive guide.

As mentioned it can be adjusted by using VCDS or similar to adjust the fudge factors and it might take a couple of tries over a few tanks of fuel to get it spot on. As far as I know dealers are unlikely to adjust it (if they can) unless you could prove it was really way off with multiple screen shots of tank averages compared to Fuelly returns.

I'm pretty anal about consumption but even I could not be fu  bothered to go that extent for a 5mpg display inaccuracy.

 

Edited 9 October, 20187 yr by Gerrycan

Not sure if mine has been adjusted - bought used, however it's usually very close to the brimmed calculated figure. Last tank was 0.2mpg pessimistic.

8 hours ago, Sangrail said:

Is that something the mechanics can do when they service it, for example? And would they recalibrate it, or simply tweak it according to my calculations based on full-tank to full-tank mileage?

 

What is the recalibration from vcds?

9 hours ago, JohnnyType2 said:

i wouldnt trust it even if it was calibrated. Used Fuelly or other similar app to get the most accurate mpg readings

I was happy with the readout my Mk1 - but it was accurate.

If by 'calibrated' we mean 'accurate' then I'd be perfectly happy with it.

 

I don't have any equipment that can do 'apps', but I am able to cope with the distance/fuel volume calculation needed to work out the consumption.

 

I'd quite like to have accurate mpg info on hand while driving, rather than waiting until the end of a tank, and then manually working out the average.

 

40 minutes ago, Sangrail said:

I was happy with the readout my Mk1 - but it was accurate.

If by 'calibrated' we mean 'accurate' then I'd be perfectly happy with it.

 

I don't have any equipment that can do 'apps', but I am able to cope with the distance/fuel volume calculation needed to work out the consumption.

 

I'd quite like to have accurate mpg info on hand while driving, rather than waiting until the end of a tank, and then manually working out the average.

 

have you got a smart phone/android/iphone device? then just download the app.

Doesn't look like there's a conclusive pattern from this thread.

 

My Mk1 1.9 TDI 110 consistently over-read by about 5 MPG (the computer's 58 MPG to a calculated 53 MPG).

 

My Mk3 1.4 TSI 140 also over-reads, but by about 2 MPG (typically an indicated 50 MPG to a calculated 48 MPG).

53 minutes ago, JohnnyType2 said:

have you got a smart phone/android/iphone device? then just download the app.

As I said, I don't have any equipment that can do apps.

 

In any case, would that not just give me the average when I fill up - because if so, I can do that anyway!

I've had my Octavia since March, at first the indicated consumption seemed to more or less match the actual, in fact I did a run from Essex to Wales with an actual MPG of 64 but the computer showed high 50s.

 

Since then the computer has been over reading consistently by 4-5mpg (I use FuelLog and Fuelly).  I also have a scangauge which plugs into the diagnostics port and relays various information including fuel consumption so I've been running that too to see what's what.  You can re-calibrate the device every fill up by entering the actual amount of fuel used but even doing that, there's still a discrepancy of a few MPG come next fill up.

 

It just leads me to believe the computer uses a "best guess" based on the various associated factors rather than actually measuring how much fuel is being used.

 

I know that this can be adjusted via VCDS but like my scangauge there's always going to be scope for it to over/under read

Yes but simple arithmetic might be beyond the capabilities or patience of some smart-phone addicts.

 

I was intigued by this Fuelly thing so I looked at the figures recorded by owners of my model and year Octavia, all of them were significantly less than the average 64.6mpg that I am currently achieving, verified by brim to brim calculation, some were as low as the lowest I ever saw on my previous Octavia taken during 15 hot laps of Croix en Ternois circuit with 3 passengers.

 

Now either the app is getting the consumption figures wrong and the notsosmart phone users dont realise or the profile of the app users drive their cars like they have just stolen them.

 

Have a look at the figures given for your vehicle and tell me if its the same story, I do realise that living where i do there are never any traffic jams but I never get to to long journeys either, my journeys are pretty much of 3 profiles, the most frequent going into town and back which is 5-7 kms round trip, to and from the running club 32 km round trip twice a week, twice a month a journey of 60-80 kms round trip  and one of 180km.

7 minutes ago, J.R. said:

Yes but simple arithmetic might be beyond the capabilities or patience of some smart-phone addicts.

 

I was intigued by this Fuelly thing so I looked at the figures recorded by owners of my model and year Octavia, all of them were significantly less than the average 64.6mpg that I am currently achieving, verified by brim to brim calculation, some were as low as the lowest I ever saw on my previous Octavia taken during 15 hot laps of Croix en Ternois circuit with 3 passengers.

 

Now either the app is getting the consumption figures wrong and the notsosmart phone users dont realise or the profile of the app users drive their cars like they have just stolen them.

 

Have a look at the figures given for your vehicle and tell me if its the same story, I do realise that living where i do there are never any traffic jams but I never get to to long journeys either, my journeys are pretty much of 3 profiles, the most frequent going into town and back which is 5-7 kms round trip, to and from the running club 32 km round trip twice a week, twice a month a journey of 60-80 kms round trip  and one of 180km.

 

It looks like when you're not logged in, the default representation is US MPG which is 10MPG less than UK MPG

That makes sense.

 

I know that I am becoming an old fart and since owning diesel Octavias drive generally for economy but I still use the road like my personal racetrack and was ashamed at the thought I was the most economical driver on the planet

5 minutes ago, J.R. said:

That makes sense.

 

I know that I am becoming an old fart and since owning diesel Octavias drive generally for economy but I still use the road like my personal racetrack and was ashamed at the thought I was the most economical driver on the planet

I did a double take as suddenly my recorded MPG had dropped from 52 to 43, wondered if I'd been sleep driving it like a race car 

1 hour ago, Sangrail said:

As I said, I don't have any equipment that can do apps.

 

In any case, would that not just give me the average when I fill up - because if so, I can do that anyway!

 

took me a while to let go of my  the Nokia 7310 too!  i dont understand you want mpg but no you don't want your average mpg? what does the maxidot reading show us? the instant reading is the most inaccurate reading on the mdot?

J.R. thats an expert bit of trolling, im proud of you. but I think you have put your decimal in the wrong place on your pad and paper or perhaps your 20 year old Texas Instruments calculator isn't functioning correctly? I always brim to brim fill and for the first few fills i measured and calculated manually and on another app to compare and they were all bang on. I could be really rude using names like luddite but i wont. Most of us dont have the time to sit down with our newspaper and mug of tea and pencil/paper so convenience and accuracy will win everytime as we live in a world driven by technology calling an app user 'notsosmart' is just daft or trolling.

the point is... that your maxidot will Never be anywhere near accurate, its too dependant on alot of external factors and calibration. 

37 minutes ago, JohnnyType2 said:

 

took me a while to let go of my  the Nokia 7310 too!  i dont understand you want mpg but no you don't want your average mpg? what does the maxidot reading show us? the instant reading is the most inaccurate reading on the mdot?

J.R. thats an expert bit of trolling, im proud of you. but I think you have put your decimal in the wrong place on your pad and paper or perhaps your 20 year old Texas Instruments calculator isn't functioning correctly? I always brim to brim fill and for the first few fills i measured and calculated manually and on another app to compare and they were all bang on. I could be really rude using names like luddite but i wont. Most of us dont have the time to sit down with our newspaper and mug of tea and pencil/paper so convenience and accuracy will win everytime as we live in a world driven by technology calling an app user 'notsosmart' is just daft or trolling.

the point is... that your maxidot will Never be anywhere near accurate, its too dependant on alot of external factors and calibration. 

I use Drivvo, it's really simple to use on the go and I can keep track of servicing as well. Then I back it up on Fuelly on the PC when I get a moment because I like the extra reports Fuelly can give you. My maxidot is generally spot on.

Definitely, to follow-up on BC deviation, you need much larger fueling data samples. I do have deviations from time to time, but in average, BC is perfectly correct.

 

Application I use is www.spritmonitor.de and I am warmly recommending it. 

If the OP has worked out that the display is about 5 mpg optimistic then surely just taking that number off the average shown for 'this trip', 'current tank' or 'long term' will give a reasonably accurate indication of consumption?

If so, and has exact deviation value, as far as I know,

- this coefficient may be calibrated through the diagnostics or VCDS.

6 hours ago, Gerrycan said:

If the OP has worked out that the display is about 5 mpg optimistic then surely just taking that number off the average shown for 'this trip', 'current tank' or 'long term' will give a reasonably accurate indication of consumption?

Absolutly right - I can do that.

 

However, I'd rather that the thing told something a bit nearer the truth. Also although I know the average over a tankful is about 5mpg out, I've no idea about the averages for shorter trips - and if a change in coding could improve the accuracy across the board, then I'd be please to have it done.

If the car's going to tell me stuff, I'd much rather it was accurate.

 

I confess I still don't know why, given it knows how many miles it's been driven and how much fuel it's used while doing so, it still manages to **** it up! I understand that there might need to be 'fudge-factors' in the electronics - but the bottom line is really not too far from those simply-measured miles and gallons.

 

It really all confirms my view that beyond a starter-motor and perhaps glow-plugs, electicity should have no role in the operation on a diesel engine. In fact given a box of 12bore blanks, and an acetylene generator for the lights, perhaps we could get rid on those too?

Edited 9 October, 20187 yr by Sangrail

7 hours ago, nidza said:

Definitely, to follow-up on BC deviation, you need much larger fueling data samples. I do have deviations from time to time, but in average, BC is perfectly correct.

 

Application I use is www.spritmonitor.de and I am warmly recommending it. 

 

Seconded re Spritmonitor - useful measurement tool. Was going to use Fuelly but couldn't find my car in the list and it seemed to only allow US gallons - Spritmonitor was easy to setup and I've used it for a few years now with my petrol Superb (see/click on signature below). However my trip computer is surprisingly accurate.

 

On the common rail diesel there are low pressure flowbacks to the tank from various points inc pump (lubrication), injectors and pressure valve. I suspect onboard computer has to estimate with what the injectors are being told to do.

 

Edited 9 October, 20187 yr by bigjohn

9 hours ago, StealthRS245 said:

I use Drivvo, it's really simple to use on the go and I can keep track of servicing as well. Then I back it up on Fuelly on the PC when I get a moment because I like the extra reports Fuelly can give you. My maxidot is generally spot on.

 

This!

 

The biggest factor for discrepancy I found was wind - stiff headwind reduces your mpg significantly.

On ‎09‎/‎10‎/‎2018 at 04:42, ahenners said:

Not sure if mine has been adjusted - bought used, however it's usually very close to the brimmed calculated figure. Last tank was 0.2mpg pessimistic.

Mine's optimistic by 0.3mpg over the last 3,000 miles.