Display mpg v Actual.
Discussion
Looking at the odometer and how much petrol you can put in is only as accurate as your odometer (ie often not very). You'd sort of hope that that figure and the trip computer figure would always match exactly, given that the ECU has all the information required to make them match (it knows how much fuel it's putting in and how much the odometer has moved on).
My car doesn't actually have an MPG readout on the dashboard, but the ECU still logs it and you can actually access it via two different diagnostic commands... which give completely different results.
My car doesn't actually have an MPG readout on the dashboard, but the ECU still logs it and you can actually access it via two different diagnostic commands... which give completely different results.
eltax91 said:
Why? Surely over a few dozen tanks the average is quite accurate when calculating from receipts?
If your odometer moves on by 200 miles, what makes you think you've actually done 200 miles as opposed to, say, 210? Even if it was calibrated properly in the first place, something as simple as a slightly flat tyre will push your odometer reading out by good few percent. I very much doubt that they are properly calibrated given that it would mean different ECU programmings for different wheel choices (since the rolling circumferences never match perfectly). ETA: It just seems odd to me that people don't trust their MPG readout, but do seem to implicitly trust their odometer?
Edited by kambites on Friday 25th April 09:00
If its a PD diesel engined car I'm led to believe anecdotally that they are not very accurate at all - mainly flatter by about 10 to 15 %. Possibly due to mechanical injectors?
Personal experience with an on board computer and piezo electric injectors are that they are quite accurate indeed - 1%.
Personal experience with an on board computer and piezo electric injectors are that they are quite accurate indeed - 1%.
Oft discussed on here. I have kept a real-world daily log of all mileage and fuel purchases for 4 different brand new manual four-pot VW Passat TDi cars, each owned for 3 consecutive years. So that's 12 years exact data. These cars have primarily been used for long motorway runs (no supermarket shopping etc.).
All have boasted average fuel consumption for these runs of way in excess of 50 MPG on the trip computer (usually 55 MPG). In reality none of them have managed more than 45 MPG during their 3 years each with me (42 - 45 MPG to be accurate).
For reference the VW claimed combined consumption for the current car is 60 MPG. To be fair each one has been a little bit more economical than the last, but nothing like the claims.
All have boasted average fuel consumption for these runs of way in excess of 50 MPG on the trip computer (usually 55 MPG). In reality none of them have managed more than 45 MPG during their 3 years each with me (42 - 45 MPG to be accurate).
For reference the VW claimed combined consumption for the current car is 60 MPG. To be fair each one has been a little bit more economical than the last, but nothing like the claims.
kambites said:
eltax91 said:
Why? Surely over a few dozen tanks the average is quite accurate when calculating from receipts?
If your odometer moves on by 200 miles, what makes you think you've actually done 200 miles as opposed to, say, 210? Even if it was calibrated properly in the first place, something as simple as a slightly flat tyre will push your odometer reading out by good few percent. I very much doubt that they are properly calibrated given that it would mean different ECU programmings for different wheel choices (since the rolling circumferences never match perfectly). ETA: It just seems odd to me that people don't trust their MPG readout, but do seem to implicitly trust their odometer?
Edited by kambites on Friday 25th April 09:00
I have a spreadsheet somewhere keeping logs on my cars for the last 6 or so years (since I started doing big mileage for work). This covers approx 180k miles in 6 different cars, ALL of them, without fail, have read 10-15% over on the trip versus the calculated brim-to-brim method.
Surely 6 different cars can't all have their ODO out by the same amount?
eltax91 said:
OK and a fair point. But surely manufacturers have more reason to make the MPG readout seem appealing, whilst there is little motivation for a ODO to not be made to be accurate?
As above, they have every reason to make their odometer inaccurate, at least slightly - different versions of the same car will have different sized wheels and the manufacturer isn't going to want the expense of producing multiple ECU images. It's not going to be much, almost certainly <5%, but it's there. Funnily enough, I was looking at this yesterday. My 172 Cup indicates about 32 mpg on the trip counter and it's actually returning 31.5 mpg when you do the sums. Pretty impressed with that. The speedo is also extremely accurate (according to my GPS).
I know the fuel economy of the 172/182 often comes up on here so I almost posted a thread about it. My commute is an absolute worst case combination of enthusiastic B-road driving and town traffic, so with that in mind I'm quite chuffed with the result. You really can coax an indicated 40+ mpg out of it on the motorway too, so it's not a pub myth.
I know the fuel economy of the 172/182 often comes up on here so I almost posted a thread about it. My commute is an absolute worst case combination of enthusiastic B-road driving and town traffic, so with that in mind I'm quite chuffed with the result. You really can coax an indicated 40+ mpg out of it on the motorway too, so it's not a pub myth.
kambites said:
As above, they have every reason to make their odometer inaccurate, at least slightly - different versions of the same car will have different sized wheels and the manufacturer isn't going to want the expense of producing multiple ECU images. It's not going to be much, almost certainly <5%, but it's there.
ALmost all cars these days have a setting in the BSI/ECU etc. for different sized wheels which is automatically configured based on the spec code which will be part of the VIN when it is entered into the ECU.I know for a fact that the wife's 307SW has the feature and that it can be altered using the official PeugeotPlanet software at the dealer (or DIY if you have the appropriate interface
).r11co said:
ALmost all cars these days have a setting in the BSI/ECU etc. for different sized wheels which is automatically configured based on the spec code which will be part of the VIN when it is entered into the ECU.
That I didn't know, I wonder if there's a way to manually change it so I can put the OH's car right for both sets of wheels? I don't have an OBC that clever.
I've given up working it out and think people that keep a spreadsheet are slightly OCD / mental. Do I get over 550 miles to a tankful? Yes? Good. No need to work the rest out really, a few MPG here and there isn't going to make me consider selling the car is it?!
As for the rest of you mad folk -
I've given up working it out and think people that keep a spreadsheet are slightly OCD / mental. Do I get over 550 miles to a tankful? Yes? Good. No need to work the rest out really, a few MPG here and there isn't going to make me consider selling the car is it?!
As for the rest of you mad folk -
C.A.R. said:
I don't have an OBC that clever.
I've given up working it out and think people that keep a spreadsheet are slightly OCD / mental. Do I get over 550 miles to a tankful? Yes? Good. No need to work the rest out really, a few MPG here and there isn't going to make me consider selling the car is it?!
As for the rest of you mad folk -
Spreadsheets....how quaint :-)I've given up working it out and think people that keep a spreadsheet are slightly OCD / mental. Do I get over 550 miles to a tankful? Yes? Good. No need to work the rest out really, a few MPG here and there isn't going to make me consider selling the car is it?!
As for the rest of you mad folk -
I use https://www.fuelly.com/
r11co said:
....
I know for a fact that the wife's 307SW has the feature and that it can be altered using the official PeugeotPlanet software at the dealer (or DIY if you have the appropriate interface ).
On this subject. Is it possible to get hold of this OBD software on the cheap? I've a mate that needs a DPF fluid low warning resetting. On my cars (VAG) you can just download VCDS lite for free and it works a treat with a standard OBD cable.I know for a fact that the wife's 307SW has the feature and that it can be altered using the official PeugeotPlanet software at the dealer (or DIY if you have the appropriate interface
).My OBC shows a consistent 58mpg.
My actual mpg is around 55/56.
A good site here:-
http://www.mpg-calculator.co.uk/
My actual mpg is around 55/56.
A good site here:-
http://www.mpg-calculator.co.uk/
LotusOmega375D said:
Oft discussed on here. I have kept a real-world daily log of all mileage and fuel purchases for 4 different brand new manual four-pot VW Passat TDi cars, each owned for 3 consecutive years. So that's 12 years exact data. These cars have primarily been used for long motorway runs (no supermarket shopping etc.).
All have boasted average fuel consumption for these runs of way in excess of 50 MPG on the trip computer (usually 55 MPG). In reality none of them have managed more than 45 MPG during their 3 years each with me (42 - 45 MPG to be accurate).
For reference the VW claimed combined consumption for the current car is 60 MPG. To be fair each one has been a little bit more economical than the last, but nothing like the claims.
Tell me, where do you keep your 'world's most boring man' award?All have boasted average fuel consumption for these runs of way in excess of 50 MPG on the trip computer (usually 55 MPG). In reality none of them have managed more than 45 MPG during their 3 years each with me (42 - 45 MPG to be accurate).
For reference the VW claimed combined consumption for the current car is 60 MPG. To be fair each one has been a little bit more economical than the last, but nothing like the claims.
Surely this is some obscure reference to Alan Partridge that I don't understand.
On a related note, if odometers are so inaccurate, why is any credence at all given to the recorded mileage of a car? Stick the wrong wheels or tyres on it and by the time it's five years old, it could be misreading by, say, 20,000 miles. Is that right?
Simon.
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