Fuel Consumption Figures; cluster and ECU.

[stefcio007]

Hi,
so the other day was testing out a new Ford Focus Sport. I was data logging through the ECU and found that the fuel reported in real time by the ECU was higher than what the cluster displayed. Sometimes even by 2L/100km difference.
My question is, why the difference? If the cluster reports a lower fuel consumption than what the ECU is reporting does this make the car look like it's achieving better fuel consumption? I mean most people won't even bother reading data from the ECU at all and will only use the cluster to get info.

Thoughts?

[theresanothersteve]

The cynic in me thinks its deliberate, makes people think they are getting better economy than they are. I'm guessing there's no mandatory obligation for 'convenience' displays to be accurate.

Wait...

...is this a cheat device ?

[Martin]

I would expect the cluster to be displaying an average over time, not the instant/current fuel flow

[stefcio007]

When testing both were set to instant fuel consumption.

The oddest part was when rolling down a hill where no accelerator was used.
Cluster said 0L/100km.
ECU said 2L/100km.
Which is right?

[feenix74]

If it was 0L/100km the engine would have stalled. I would expect the ECU figure to be the more accurate in that instance.

[JustCruisn]

If it was 0L/100km the engine would have stalled. I would expect the ECU figure to be the more accurate in that instance.

The ECU cuts fuel to zero. Engine is still running but being turned by the transmission.
Rolling in neutral uses more fuel then in gear.

For OP id say that ECU is reading +2 on actual.

[dinot81]

For more accurate reading that the what the cluster is saying, fill your tank to the cap, set trip meter to zero, next time fill up again, fill to the cap and work out what the actual consumption is. I do that on mine just for fun.

My last fill was 55 litres with 920kms done:

55
----- X 100 = 5,98L / 100kms
920

[stefcio007]

Thanks for the tip, I already have years worth of data collected like that but for another car.
I'm interested as to why there is a discrepancy at all between the two values since they are supposed to be showing the same thing.
I'm going to do further testing on other models to see if I can obtain similar results.