Topic: fuel consumption computer

[bigsilver]

I am loving my 07 4runner that we picked up in feb. However I have noticed a difference between what the computer says in
litres per 100k and what i am actually getting. For the last few fill ups I reset the trip log at 0 and then when i fill up I take my
total km's divide by the litres for the fill up and X by 100 and it tells me that i am using about 1 litre per 100k more than what the
computer says. Are these fuel consumption computers on vehicles these days supposed to be fairly accurate or are they there as
as general guidline because i didn't think the difference would be that much or am I calculating it wrong.

Any help would be grateful

Thanks

Bigsilver

[articsteve]

Vehicle fuel consumption computations/computers are a toy to amuse ppl on long trips. 

[blur911]

The trip computer on my 20 year old Audi is quite accurate. 

Does your Toyota have OE sized tires on it?

[johngenx]

The computer in our Corolla is pretty accurate, typically within 0.2-0.5 L/100km average economy over a trip.

[Weexy]

The computer on my TL is a bit optimistic, especially if i've done mostly city driving. 
I did the math last week on a partial fill up and figured about 12.5L while the TL said i'm getting about 11 (I do reset the computer on most fillups)

When I do mostly hwy driving, it seems pretty bang on though

[blur911]

The computer on my TL is a bit optimistic, especially if i've done mostly city driving. 
I did the math last week on a partial fill up and figured about 12.5L while the TL said i'm getting about 11 (I do reset the computer on most fillups)

That reminds me, I once had a car (Probe GT maybe?) in which the average Liters/100km seemed more like a "running average" than as if calculated from the last reset.  Perhaps they don't all work as we think they might?

[madjak30]

They do not take an average from the last reset, but they average over the last 100kms.  It may be further, but not much.  I was noticing the same thing.  I would reset it everytime I filled up, and it never matched what I calculated. 

I haven't reset it in the last ten or so fill ups and it still reacts quite quickly after driving in the city for a while, then going for a couple hours down the hi-way.  They must average over the last x-number of km.  My truck is showing 10.8L/100km right now, but I bet if I filled it and did the calculation it would come out closer to 13L/100km.  So, take the info with a grain of salt...and trust your calculated numbers more...

I was towing a 5000lb trailer yesturday (14.2L/100km), and on the same tank after an hour down the hi-way it was telling me I averaged 11.8L/100km...all within 500kms.  You be the judge.

[X-Traction]

These mileage computers usually are connected to the fuel injection metering system to determine immediate consumption.  They can be averaged over varying amounts of time.  If there is no averaging, such as on the '06 Grand Vitara, they are little more than a curiosity.  If they're averaged over too long a time, you may as well just use the average trip mileage function.

The trip computer on our Spirit averages the immediate readout every 4 seconds.

I'm of the opinion that the fuel injection metering system is very precise, so immediate consumption readouts should start with very accurate information, which is then diluted, but not degraded, by averaging.  They also use whatever system the car uses for the odometer, to determine the distance being travelled.  I suppose that is subject to a bit of error.

Typical trip/mileage computers also have a reset function and give an average mileage since the last reset.  Since this uses the same signals as the immediate mileage calculation, it should also be pretty accurate.  The calculation is done and the display updated at different time intervals for different cars.  I believe the interval on the Spirit is 18 seconds.  So the longer you've been driving, the more slowly the readout changes.

Then there's the DTE - Distance to Empty.  If DTE used the fuel injection metering signal, it would somehow have to account for fillups because it has to know how much fuel is in the tank before it begins deducting the fuel being used.  Typically DTE is just dumb and uses the same signal/current as the regular old fuel guage.  We all know how inaccurate those things are.  So you can have 10 litres left and DTE says you are out of fuel.

On our car, DTE is set up this way and is basically useless.  The "trip computer" readout also has the time and distance since the last reset.

If anyone knows of cars where DTE is accurate, and how it is done, I'd be interested in an explanation.

[johngenx]

I have no idea of how the DTE is calculated in our Corolla, but it's very accurate.  The countdown in mileage is to the 42L consumed mark, which is where the fuel warning light comes on.  Sure enough, as it gets to zero km remaining, the light comes on, and invariably if I fuel up quite quickly, 42L is the fuel required.

[X-Traction]

Agreed, because the DTE uses the same information as the fuel gauge, which incorporates the low fuel warning.

But, I looked forward to having DTE so that on a long trip I could decide whether to fill up at the next town, or wait until the next one after that.  I know that if I fill up when the fuel gauge reaches "Empty" and the low fuel warning lights up, the fillup will be nowhere close to the capacity of the tank.  Typically there are 10-15 litres still in the tank. 

So "Empty" doesn't really mean empty.  And DTE reaching zero doesn't mean I'm out of gas.  Which doesn't really make sense and which means that at 8l/100km, I could gone, say, another 150km before filling up.  Which I mistakenly thought DTE would allow me to figure out.  And is why I never use the DTE function.  It tells me nothing the fuel gauge doesn't, but in greater detail

I've heard these things can "learn" where the real zero is.  The theory was that if DTE reached zero and you kept driving, it would use the consumption until fillup to recalibrate itself.  You'd think this would be possible and not too difficult to program.  But alas, the thing never learns.

Where I live, gas prices fluctuate a lot.  The same reasoning can be used to delay fillups for lower gas prices.

[Guy]

The computer on my TL is a bit optimistic, especially if i've done mostly city driving. 
I did the math last week on a partial fill up and figured about 12.5L while the TL said i'm getting about 11 (I do reset the computer on most fillups)

When I do mostly hwy driving, it seems pretty bang on though

Same here.