[Snowball]

I am wondering how accurate is the fuel consumption figure given in the multi-function display (MFD) in the dash panel?

I have recently changed my vehicle for one of an identical model and(except for going from a 7-seater to a 5-seater), the specifications for the vehicle, a VW Touran, are identical. The first vehicle had around 34,000 miles on the clock, whereas the current one has not yet reached 2,000 miles.

The new car is giving a reading that indicates an approximate increase in mpg of about 6 to 8 mpg. Could this be because the engine of the new car (1.9 TDI) has not yet fully loosened up, a variation in instrument characteristics, or a combination of both?

The downward climb of the fuel gauge behaves differently (at first I thought it was sticking at the half empty mark), and I wondered if the fuel consumed in the MFD is measured by the ECU, or simply from the fuel gauge. If it is the latter, could any changes in the MFD readings be at variance to the true mpg usage?

Has anyone else met with similar findings?

Regards, Snowball.

[wagolynn]

How about tyre size and pressures, are they different?

My last working car was a Ford Fiesta with the Peugeot 1.4 diesel in, with alloys hence silly wide tyres and MPG indicator, I noticed with that 2 PSI down on pressure would increase consumption by 0.5% to 1%.

But 2000 miles is not much distance for the new engine, even though running in is not required.

[Snowball]

[QUOTE=wagolynn;40081]How about tyre size and pressures, are they different?

But 2000 miles is not much distance for the new engine, even though running in is not required.[QUOTE]

No difference; even same make and type.
The only difference is that this engine is common rail, but I don't see this as being significant if all other specs are the same, including the claimed performance figures.

I realise that the engine may still be tight; they reckon these can take up to 10,000 miles to completely free off. TBH, I cannot remember what how the cars compared new-against-new.

I am getting slightly higher fuel consumption when towing, compared to my previous car, just prior to selling it, and the same caravan. But I cannot compare it to when the last car was new, because the van we had at that time was 1,100 kg lighter.
Not that there is a problem, of course, but with figures to compare, interest tends to send me down the road of inquisitiveness. Sad, ain't it?

P.S. Wish you'd stop showing off about the ageing process; I'm trying to ignore it.

Regards, Snowball.

[MerseyPatrol]

The vw engines are built pretty 'tight' and it will be another 10,000 miles before it is ran in.

[Snowball]

I thought I had heard somewhere that it could take 10,000 miles before a diesel engine is "fully run in".
A bit different to the old days; I can remember thinking about a top overhaul and valve regrind around this mileage.

Does the ECU measure the fuel consumption shown in the MFD? I would imagine that the tank sender unit would be too crude for this to be reliable; particularly as the reading indicates in increments of 0.1 mpg.

[wagolynn]

Originally Posted by Snowball

[
P.S. Wish you'd stop showing off about the ageing process; I'm trying to ignore it.

Regards, Snowball.

Unfortunately I haven’t found a cure yet, I am trying though, Mrs says very trying.:o

[Snowball]

Originally Posted by wagolynn

Unfortunately I haven’t found a cure yet, I am trying though, Mrs says very trying.:o

Hmm! Sounds familiar.

Back to topic; I have read somewhere that these MFD readings are not very accurate, but no reasons for stating this, or levels of inaccuracy were mentioned.

My own thinking is that, as the engine management system has to meter the fuel quite accurately to achieve stable performance, and the distance covered (mileage) is also reasonably accurate, then the calculated mpg in the MFD should be reliable.
I suppose, depending on the number of "steps" in the programme to achieve this, and the probable tolerances that must exist in the manufacturing of the system, then the build up of the tolerances could possibly result in a meaningful difference of the mpg reading between similar vehicles.

Does that make sense?

[Hometune]

I don't suppose you know the engine codes for both cars by any chance? On your present car you should find it on the cam cover as a 3 letter code something like AWX or on a sticker in the spare wheel well. It may be possible from the codes to offer an explanation. Common rail is more economical than the older system due to the much higher pressures.
If you watched the edition of Top Gear where they each drove a car from Switzerland to Blackpool on one tank of fuel, you will have seen that the display reading was well out. In fact Clarkson drove around another 30-50 miles in a Jaguar 2.7 twin turbo diesel Jaguar even though the display said the tank was empty.
Incidentally, in a lot of new cars the fuel gauge can be adjusted by a scanner for a more accurate reading.

[wagolynn]

Originally Posted by Hometune

I don't suppose you know the engine codes for both cars by any chance? On your present car you should find it on the cam cover as a 3 letter code something like AWX or on a sticker in the spare wheel well. It may be possible from the codes to offer an explanation. Common rail is more economical than the older system due to the much higher pressures.
If you watched the edition of Top Gear where they each drove a car from Switzerland to Blackpool on one tank of fuel, you will have seen that the display reading was well out. In fact Clarkson drove around another 30-50 miles in a Jaguar 2.7 twin turbo diesel Jaguar even though the display said the tank was empty.
Incidentally, in a lot of new cars the fuel gauge can be adjusted by a scanner for a more accurate reading.

HI Hometune, do they actually meter the fuel separately or do they try and calculate from injector times? I don’t think the tank fuel gauge is sensitive enough is it?

[Hometune]

Sorry, don't know this one. But it must include the vehicle speed sensor, possibly throttle position sensor, airflow sensor and coolant temp sensor. The last 3 control injector opening time. Someone must know for certain - Wikipedia?

[Snowball]

I go along with wagolynn's opinion that the tank gauge isn't sensitive enough.
When I thought the gauge in my present car was sticking at the half-empty mark (different behaviour to my previous, similar model), the service dept said it is only a "guide".

If the other instruments in modern cars are as accurate as we think they are, isn't it about time the fuel gauge/sender unit caught up with technology? Particularly if, as I believe, the MFD "low fuel" warning gets its message from the same circuitry.

[wagolynn]

Originally Posted by Hometune

Sorry, don't know this one. But it must include the vehicle speed sensor, possibly throttle position sensor, airflow sensor and coolant temp sensor. The last 3 control injector opening time. Someone must know for certain - Wikipedia?

Re-measuring fuel used for MPG display. It looks as though the amount of fuel used comes from the signal to the fuel injectors. The flow rate of the injectors is known so computer measures length of time on. That’s all I could find on the subject. But this means a faulty injector makes MPG useless. I will stick to logging fuel and mileage. I use it as a guide to health of engine and weight of right foot.