VW Golf GT 2.0 TDI (2004) - Real world MPG v trip computer (is mine faulty?)

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Mon 18 Apr 2016 20:18
VW Golf GT 2.0 TDI (2004) - Real world MPG v trip computer (is mine faulty?) - K21000

After looking at various posts, I have come the conclusion that trip computers generally seem to be out by up to 10%. I therefore think there must be a fault with my trip computer MPG reading! I have owned and driven the car for nearly 3 months and reset the overall average 3 times. I do mixed A/B roads, not much stop/start (rural Cornwall). Commute is 18 miles to work each day, but drive weekends with longer journeys currently averaging overall 280 miles a week. My cars average real life MPG (Honest John) is supposed to be 49. My reading fluctuates between 58.9 and 58.7. I have run the car only on Shell V Power diesel since owning it and have done way over 2k miles with this fuel. The car has been properly serviced and maintained by the book and has 60k miles on it. I have not done a proper brim to empty test yet, but wondered if other people's trip computer MPG's are as way out as what mine seems to be?

Edited by K21000 on 18/04/2016 at 20:19

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Mon 18 Apr 2016 20:34
VW Golf GT 2.0 TDI (2004) - Real world MPG v trip computer (is mine faulty?) - focussed

You cannot check your actual mpg by referring to your onboard computer or HJ figures. Nothing wrong with HJ real mpg figures, but they are not for your car, on your fuel, driven by you.

Do a brim to brim and then you will know for certain.

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Mon 18 Apr 2016 22:25
VW Golf GT 2.0 TDI (2004) - Real world MPG v trip computer (is mine faulty?) - Engineer Andy

I concur with Focussed - trip computers aren't really that accurate - mine certainly is about 10% out, and using the brim-to-brim method (and over a reasonable period) I'm averaging 41-42 mpg in my Mazda3 mk1 1.6 petrol, actually 10% more than the combined mpg the manufacturer states (about 38 [that stated on this website is for the mk1.5 not mk1) and more than that in the 'Real mpg' section.

The type/length of journeys, traffic levels (i.e. time of travel is very important) and especially driving style (I am generally light-footed, though no 'Captain Slow' and generally make longer trips on emptier roads) can make a huge difference to the fuel usage.

For you, diesels are far more efficient once warmed up than petrol cars, so there is likely to be a far higher range of values recorded in HJ's Real mpg section for diesel cars, given some do mostly short journeys in heavy traffic, others mainly fast A-road/motorway cruising at 50-70mph.

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Tue 19 Apr 2016 01:16
VW Golf GT 2.0 TDI (2004) - Real world MPG v trip computer (is mine faulty?) - Avant

"I have not done a proper brim to empty test yet..."

That's what you need to do (brim to brim anyway - you don't need to run it right down to empty). It's the only way to be sure: odometers are generally more reliable than trip computers.

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Tue 19 Apr 2016 07:50
VW Golf GT 2.0 TDI (2004) - Real world MPG v trip computer (is mine faulty?) - RT

"I have not done a proper brim to empty test yet..."

That's what you need to do (brim to brim anyway - you don't need to run it right down to empty). It's the only way to be sure: odometers are generally more reliable than trip computers.

That's the bit that's always baffled me - car distance is measured from the wheels, using the same data for trip computer and odometer display - the fuel used is metered with incredible accuracy by the fuel injection system so the total fuel used should be very accurate - there may be a little evaporation but that's negligible with diesels.

So why can't trip computers calculate accurately?

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Tue 19 Apr 2016 10:43
VW Golf GT 2.0 TDI (2004) - Real world MPG v trip computer (is mine faulty?) - RobJP

You are assuming that the fuel used is measured from the metering system.

It is not, it is measured - very crudely - by approximating the accelerator pressure switches to the amount of fuel being used.

As an example, if you were 'labouring' a car in too high a gear, you might have the accelerator pressed quite firmly, but the car wouldn't be delivering the fuel requested to the cylinders. If, on the other hand you were cruising down a long slope on the motorway, then the very light throttle might not even register as fuel use.

Interestingly (or maybe not) a number of cars actually have methods for altering the coding on the trip computers to change how optimistic / pessimistic they are.

Myself, I largely ignore what the trip computer says. I zero the odometer each fillup, and do a very quick mental calculation as to fuel used for each tank (should be 43-45 mpg). If my numbers started to change drastically then I'd investigate further. If no change then I just carry on.

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Tue 19 Apr 2016 11:05
VW Golf GT 2.0 TDI (2004) - Real world MPG v trip computer (is mine faulty?) - RT

You are assuming that the fuel used is measured from the metering system.

It is not, it is measured - very crudely - by approximating the accelerator pressure switches to the amount of fuel being used.

As an example, if you were 'labouring' a car in too high a gear, you might have the accelerator pressed quite firmly, but the car wouldn't be delivering the fuel requested to the cylinders. If, on the other hand you were cruising down a long slope on the motorway, then the very light throttle might not even register as fuel use.

Interestingly (or maybe not) a number of cars actually have methods for altering the coding on the trip computers to change how optimistic / pessimistic they are.

Myself, I largely ignore what the trip computer says. I zero the odometer each fillup, and do a very quick mental calculation as to fuel used for each tank (should be 43-45 mpg). If my numbers started to change drastically then I'd investigate further. If no change then I just carry on.

Seems a silly waste of good electronic data.

My Vauxhall from 2000 was consistently 8% optimistic - but subsequent Subaru, Hyundai and now VW are all over the place making a single correction factor not possible - the Hyundai could vary from +2 to -13% !

Edited by RT on 19/04/2016 at 11:05

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Tue 19 Apr 2016 12:01
VW Golf GT 2.0 TDI (2004) - Real world MPG v trip computer (is mine faulty?) - gordonbennet

Interesting point about odometers, if the speedo is up to 10% optimistic as many are, wouldn't the odo be the same.

Has anyone compared sat nav recorded miles (do they even do this?) with their car's.

The fuel computer in modern lorries is accurate, well all the ones i use are, almost identical figures to the mpg readings at our pump where i work which displays mpg since last fill.

I've often wondered if it might be of interest to BR'ers if on my regular trips i noted mileages at say M1/M6/A14 Cattthorpe junction or M1 jct 15 (both of which i use nearly every day) to a few motorway junctions going via M1 M6 and M5/M50 into Wales via M42, or M1 going south from jct 15, all my reguar jaunts, would noting those mileages be of any interest to those here to compare a calibrated speedo/odo with the one on their car just for the hell of it? if so just say so and i'll make some notes....maybe some others in different areas with calibrated speedos could do the same, we might unearth another Dieselgate between us..:-) could be interesting.

Edited by gordonbennet on 19/04/2016 at 12:04

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Tue 19 Apr 2016 13:17
VW Golf GT 2.0 TDI (2004) - Real world MPG v trip computer (is mine faulty?) - RobJP

Geek that I am, I've measured various journeys and distances using different methods over the years. From motorway marker posts (was easier when they were in miles, not having to re-calculate them in my head whilst driving) to google earth measuring tools, to various website route mapping tools, or onboard (built on or standalone) satnav systems. Comparing all of the above with recorded miles on the car has always brought minimal difference.

One example in particular. A place we stayed in near Aviemore, and stayed there again the following year.

Year 1. Audi A6 avant. Online routefinder gave it as 415 miles. Recorded 418 miles.

Year 2. BMW 3 series with satnav. Route calculated at 415 miles. Recorded 420 miles.

Both of those trips would have a small amount of additional mileage due to service station stops (I'd guess under half a mile added)

In both of those cases, we are looking at roughly 1% variance.

Whilst speedos CAN be up to 10% out, I'd be frankly astonished if many were anything like that.

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Tue 19 Apr 2016 19:07
VW Golf GT 2.0 TDI (2004) - Real world MPG v trip computer (is mine faulty?) - HandCart

You appear to have been lucky, then.

A journey I do frequently has traditionally measured about 195 miles on my cars' odometers,
but Google Maps says it is 189-190.
Just did it the other weekend in the car recently acquired for SWMBO, and I could swear that said it was only about 180!

Didn't check it on the way back, but I have noted that there are at least 3 manufacturer-defined wheel/tyre size options for that car, and just going between the 175/65x14 versus 185/55x15 versions results in a 2.86% difference in theoretical overall radius anyway. That would be 11.4 miles difference measured on the odometer on a 400-mile journey just due to that alone.

;-)

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Tue 19 Apr 2016 14:02
VW Golf GT 2.0 TDI (2004) - Real world MPG v trip computer (is mine faulty?) - RT

Any error in the odometer calibration will be reflected in any brim-to-brim calculation - as indeed will the error caused by tyre wear over time.

I have checked a 2000 Astra's odometer against the motorway distance posts on the M6 for 180 miles - the error was less than the variation caused by tyres wearing down from 7mm to 2mm.

I always make a point now of checking my car's speedo against satnav, 73 GPS = 75 speedo currently - but that's on summer tyres, the differently sized winter tyres giving 72 GPS = 75 mph

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Tue 19 Apr 2016 17:39
VW Golf GT 2.0 TDI (2004) - Real world MPG v trip computer (is mine faulty?) - madf

Tyre wear means odometer readings are never corect.. A new tyre is a bit longer in circumference than a worn one.

And we all know legally speedos must over-read so odometers do as well..

And fuel on hot days expands so you get less bang for your litre versus cold days.

Too many variable - as anyone who has done O level physics and thought about the subject would realise..

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Fri 22 Apr 2016 12:08
VW Golf GT 2.0 TDI (2004) - Real world MPG v trip computer (is mine faulty?) - industryman

I had one of these cars for three years as a company car doing 700-800 miles per week and kept brim to brim records for every drop of fuel I put in it. Over 110667 miles it averaged 51.3mpg running on the cheapest supermarket diesel I could find. When I got the car the the fuel consumption meter was about 12% optimistic. I mentioned this to the VW dealer at its first service (18000 miles) and they responded that it was not an accurate device but meant to give an approximate guide as to fuel use. I accept this, but said to them that 2-3 mpg out was reasonable but I thought 5-6 mpg over the top was excessive. I added that in my experience, electronic devices like this usually had a simple method to adjust calibration. They then agreed to consult VW but with no promise of improving it. The result was that from then on the reading almost exactly matched my brim to brim figures. So it can be done although I think it was pure luck that the code change they entered into my car's electronics gave the perfect answer.

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