Computed fuel consumption - Andrew-T
In a recent thread the accuracy of on-board consumption was questioned. My experience with SWMBO's Clio 1.6 16v suggests that its figures are (at a guess) 15% optimistic. On longish trips using a light throttle the indicated figure settles at about 52mpg, but ACTUAL consumption over 1000 miles is 40-42. I can rationalise this as follows.

The car measures miles within 2%, but has no accurate idea of fuel quantities - it can only guess on the basis of the float in the tank, and those are notoriously vague. Whether by accident or design, gauges tend to be pessimistic in the lower half (i.e EMPTY means about 3 litres left and HALF probably means nearer 60% full) and that is the range in which our gauge normally works. Therefore the car 'thinks' it is using 25 litres while it is actually using nearer 30.

However the display is continuously responding to throttle opening, as consumption immediately increases when climbing hills, so clearly it is metering the fuel injected - but it doesn't know precisely what the quantity is. Can anyone out there throw light on the method?
Computed fuel consumption - jud
Andrew i always calculate mpg by the old tried and tested brim to brim test and if possible used the same pump. The computer is always within 1mpg and often near bang on. Both i assume see the same "miles" input so i can only assume that one of your other "inputs" required for the calculation must be inaccurate.
Are you sure that you are not looking at the instantaneous mpg reading?
Computed fuel consumption - Andrew-T
jud - there is only one consumption readout on the Clio, which is not 'instantaneous' but cumulative, restarting whenever the tripmeter is reset. The longer I wait before doing this, the less the figure changes - is calculating over an increasing period. But unless it has been going for 50 miles or more, climbing a steepish hill is enough to produce a noticeable change. It seems to update every second or two.

As it cannot know in absolute terms how many litres are in the tank, it can only calculate by feedback from the fuel injectors, unless there is a flowmeter somewhere? I suspect that this device (whatever it is) is wrongly calibrated.
Computed fuel consumption - Peter D
The quantity of petrol injected into you engine is controlled by the management unit which depending on the demand, i.e.throttle, air flow meter and lambda sensor the product of which controls the duration of the pulse that opens the fuel injector.. This pulse width and the rate it fires ( RPM ) provides the measure of quality and the electronic speedo taken from the final drive shaft give you distance travelled thus base line for the computed fuel consumption. To stop the numbers jumping all over the place the results are dampened significantly. The amount of fuel in the tank is not used for the calculation of MPG

If you had cruise control ( thus removing the human right foot element ) and travelled a 100 miles on a relatively clear motorway the computed fuel consumption and the actual can be very close.

You are now down to the tyres. If new then they have a rolling distance some 1 to 1.5% bigger than there worn value. Add to this you may have fitted alternative alloys and different profile tyres and you may have another correction. Some vehicles computer/EMU have a programmable element for the build of the vehicle and the Alloy/Profile of the rubber.

Lets be honest it is only an indication and the full tank to full tank is the real situation. Having said that I have heard of people noticing a change in the computer MPG that caused them to call into the garage to discover there O2 ( Lambda ) had failed and the car was overfuelling. Why he didn’t see the EMU/Lambda light on as well I do not know. So it can be another diagnostic tool. Regards Peter
Computed fuel consumption - Andrew-T
Thanks Peter et al. Although we have not owned the car from new, there is nothing to suggest that it is running incorrectly or that the wheels are not as programmed. Perhaps its first MoT in June will show an odd lambda value. But I presume that although the electronics do an accurate job of maintaining mixture and timing, it does not follow that they know in absolute terms the volume of each dose to the injectors.
Computed fuel consumption - pmh
I do not know what flow meter technology Renault use for the Clio, but since the ECU is responsible for delivering the fuel in correct quantities, it must therefore be aware of the instantaneous (and hence by calculation, historical cumulative) figures. This must be the data source for display calculation unless they install a duplicate set of sensors.

In the Old mechanical K jet systems the figure was calculated from the measured depression (vacuum sensor). Since the injection quantity is determined by the Mass flow sensor (which will have a defined relationship with inlet flow and hence depression) the consumption can be computed. My understanding of this system is that it will only give correct readings if the 'mixture adjustment' is set up correctly. ie the consumption figure is not an absolute measure. That apart my Golf |Mk2 with K jet gives a reading within 2% of brim to brim measures.

Can anyone comment on or correct this view?

pmh (was peter)
Computed fuel consumption - pmh
When i started my post, the Peter D post was not there!
Apologies for duplication etc etc.

pmh (was peter)
Computed fuel consumption - SjB {P}
Agree with Peter D and pmh.

I had the dispay unit (infamous \'missing\' pixels problem) replaced in the Vectra SRi V6 Estate I drove just over three years ago, and the new unit resulted in inaccurate fuel consumption. (40 MPG at a steady 70 MPH, I wish!) On going back to the dealer, they explained that the unit was programmable, and used fuel flow rate for the injectors as the basis of calculation. Five minute\'s worth of TEC2 plug-in, and the result was a unit that gave exactly the same readings as the one on which the readout failed, and which was pretty close to spot on with the full tank to full tank checks that I made.

Computed fuel consumption - eMBe {P}
The precision way to measure consumption is to use distance/fuel quantities which are measured with fine accuracy, say to British Standards. Using milometer (or road markings) for distance, and forecourt pumps (or your tank) for fuel amount, will give you acceptable estimates but not precision.

The car computer will use its air/fuel mix control system to calculate the fuel used. Similarly it will calculate distance travelled. Neither of these will be precise, being subject to calibration errors.
Computed fuel consumption - Andrew-T
MB - on the question of precision, it should be possible to rely on less than half a percent at the pumps (0.1 litres in 20). Does anyone know how accurate the motorway 100-metre posts are (of course one has to compare over 30 or 40 km)?
Computed fuel consumption - Cyd
On my Rover SD1 the trip actually had a fuel flow sensor which measured the flow rate to the carbs. This was actually quite accurate (usually within 2mpg) but is old hat now.

My Rover 827 had a trip computer enabled to read out instant and average fuel comsumption. It measured the fuel flow rate by measuring the time the injectors were held open at each 'squirt'. It was very accurate by dint of monitoring all six injectors and summing the individual times. In theory the fuel admitted at each squirt is directly proportional to the time the injector is open. The 827s computer was always within 1mpg.

On modern, cheaper cars with computers, I cannot state as fact but think that the computer monitors just one injector. It is possible that under constantly varying driving conditions the other three injectors could have been held open for a different time than the one that is being monitored, hence less accuracy. However, 15% is very poor accuracy and I for one cannot understand fully why it should be so innaccurate.
Computed fuel consumption - googolplex
Whether by accident or design, gauges tend to be pessimistic in
the lower half (i.e EMPTY means about 3 litres left and
HALF probably means nearer 60% full) and that is the range
in which our gauge normally works. Therefore the car 'thinks'
it is using 25 litres while it is actually using nearer

My experience is quite the opposite in Fords and Vauxhalls - I always brim the tank and find that the 2nd "half", according to the fuel gauge, drains away far quicker than the first.

Computed fuel consumption - Matt
I agree with Splodgeface about the above. Never thought I'd write a sentence like that!
Computed fuel consumption - Andrew-T
Spl - I've only owned one Vauxhall (1984 Cavalier) and no Fords, so can't comment. But I seem to recall many gauges which would read FULL, then after 20-30 miles would start dropping fairly fast to around HALF, then more slowly until a more rapid drop just to warn you that you may run out quite soon (though in fact you won't).

Value my car