Fuel consumption figures computer wrong

My board computer calculates 11 (L/100 km) But the real consumption of fuel is 12.5 L /100 KM. Why does the board computer calculate wrong?

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Try resetting it.

Kristian

How do you do that?

BR:Z

No, it is not. My Saab's computer gives mpg figures that are less than 1% offset.

BR:Z

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Consult your owners manual. Type of car isn't mentioned here. In my 9000i, I simply press and hold the R button on the computer for 4-5 seconds. My owners manual says the computer should be reset at least every 3000km, could be the same for other models.

Kristian

The SID is an average mpg which might be based on a long duration. It's time for a reset. Tell us which car you have and we can advise you on what to do.

then you're lucky or manage to drive identically every single time!

I haven't reset mine in a couple of thousand miles and it says 31.2mpg. Of course that's averaged over that 2000 or so miles so if I now go and drive hard for half a tankful, i'd expect a much higher error than 1.5 litres per 100km.

Similarly, resetting it for every journey (unless they are significant in distance )isn't accurate because you're filling a small amount for a small mileage and filling from a pump whose accuracy is unknown to you and could introduce errors and you can't be guaranteed of the click off point of the pump either.

Like I said, either you're lucky or drive identically, either way, the display is a calculation which is averaged and therefore can't be taken as absolute.

David.

What's wrong with resetting the computer when you fill up and comparing the reading with the pump reading at the next fill-up? If the OP was doing that, then the discrepancy of over 10% is very dissapointing.

Colin.

Yup. 10% too good a reading here :-) (9000, 1997)

I think they are a complete gimick,

Regards Charles

Are you resetting the average every fillup and still finding this discrepancy, or are you comparing a one-tank average to an overall average? Two different things, of course.

The two times I bothered to check mine ('99 9-5 with 2.3 LPT) it was dead on. Haven't checked since.

Dave Hinz

Nothing wrong with doing that except that a) you don't know the accuracy of the pump and b) can't guarantee that the fuel cut off works exactly the same the next time so discrepancies in the filling aren't being taken into account.

I just can't get hung up on that amount of difference. :)

David.

Pumps should be reasonably reliable - I'm pretty sure they are legally regulated. It won't be perfect, but you'd have to be going some to get it wrong by over 6 litres in a tankful. Adding up all the uncertainties I'd expect well under 5%. For as much as 10%, I'd be starting to worry that something was wrong

- maybe the fuel rail pressure is a bit high or something.

Colin.

For volume, yes. But switching off when they feel the tank is full? Probably not, which is what I think David was getting at.

It's watched in the US, but people have been busted for it. Couple of years ago, one of those investagative TV programs found that some stations in large urban areas were playing with the numbers on the pumps, which was of course followed by industry people saying it can't be done ;)

Tire diameter maybe? If the tires are significantly different than what the car shipped with, that'll throw the distance part of the equation off as well. If it's a, say, 7% error, that's a lot when it comes to fuel, not so much when it comes to tires. A GPS could be used to measure overall distance, if it's all within view of the sky.

Dave Hinz

Assuming that the car calculates fuel consumption from * how long the fuel injectors are open * the fuel pressure * how much fuel will flow through the fuel injectors per time unit * the distance driven and that the driver calculates fuel consumption from * how much fuel he/she bought * distance read from odometer then any error caused by tire/tyre diameter will change the calculations done by the car and the calculations done by the driver with the same amount, i.e. the tires/tyres doesn't matter when comparing the fuel consumption numbers for this car only. The tires/tyres sure do matter if comparing fuel consumption numbers between different cars, but then the driver introduces much more variation in the numbers.

Error sources that affect calculations done by the driver:

• Station pump shut off point. * Station pump volume measurement accuracy. * Fuel temperature during fill up (volume changes by temperature).

Error sources that affect calculations done by the car:

• Fuel rail pressure. * Fuel injector (volume per time unit accuracy). * Fuel injector (the difference between electrical signal and the actual mechanical open/close times are not what ECU expects). * Fuel temperature.

Error sources that affect both calculations done by the driver and by the car:

• Tire/tyre dimensions (different brands or size). * Tire/tyre dimensions (wear)

Quite.

We already get overcharged and nobody does anything about it. Minimum delivery on most pumps for payment purposes is 2 litres, next time look at the pump price per litre, say 80.9, multiply by 2 and you get 161.8. You'll be charged 162 which is absolute overcharging, you haven't received that amount of fuel and it's not *my* fault that we don't have a 0.8 pence coin!

I thought of tyre diameter but then surely it would cancel itself out because the odometer input is based upon revolutions and so the apparent distance that the car has travelled will be the same reading the driver will use to measure fuel as the distance the computer will use to calculate the same figure.

David.

There's one aspect of this fuel consumption discussion that I've not seen mentioned: assuming the metering is 100% accurate, what use is the figure? I mean, who of us can be sure of being able to replicate, or even control to the same degree of accuracy, the conditions of our subsequent driving? A few percent either way are probably not worth the brain-ache. ;-)

Though, yes, it's interesting as an academic exercise.

-- Andrew Stephenson

recent het gemiddeld verbuik nog gereset?

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Looking for changes in fuel consumption may be useful. A new air filter in my Saab improved consumption by a couple of mpg. Also useful for comparing what you get from keeping a constant speed in contrast to white-knuckle driving. Also remember that fuel is taxed a lot more on this side.