Low gas consumption :-(
 

I really need to get a SuperMid for my swift. !

It has been 2 months since my last tank fill and I still have another 100km to go before I can fill up again and calculate my latest whole tank FE figure.
If I fill at half a tank or a quater I am more likely to get pump fill errors and a worse average compared to full fills.
If only my car had excessive fuel consumption , then it would be easier to see it and reduce it.

Man that sounds so whacked :D

Frequent filling shouldn't affect your overall (running) average.

(Unless you're saying that the pump itself is the problem, vs. inconsistencies in the level you fill to in your tank.)

MetroMPG

Yup , inconsistant fill amounts can fuzz the numbers.

Doesn't it all average out in the end? If you underfill this tank and calculate extra-good-high MPG, your next tank will take a little more to fill, making its calculated MPG lower. But your cumulative MPG will be accurate, won't it?

MetroMPG -¨Doesn't it all average out in the end?¨

Not really ... Only if you can be assured that you are filling up the exact same level every time.
I fill up the filler pipe to the very top , but there still may be pockets of air trapped in the tank.
This could be as much as a litre of air instead of gas.

If my small top up fill was only 5 liters in total then I could have an error of 25%.
That , or whatever error amount over 1 whole tank full of 35 litres would give a possible error of only 3%.

If because of the shape of the tank trapped air is present when filling up you could get a variation depending upon how much of this air that is purged.

I had 1 car (many moons ago) that if you filled it up rite to the top and then rocked the side of the car violently a few times the fuel would drop in the filler neck by at least 3 litres. (conservative , I think 5 litres)

Because of this possible variation you never knew if full is trully full.

I didnt do it often (only for trips) because it looked like I was humping my car.

Car Porn
 

ROTFLOL! :D


RH77

Also ,,if my memory is still good ..on some cars (i know Nissan did this) they used an extra small tank , of about 10 litres in size to take fuel when it expands when heated.

It was a small tank mounted above the main fuel tank fed by tubes to the main tank.
Its not intended to be filled with gas when filling up , but it does take a little.

You would never get accurate fill volumes as sometimes this extra tank would take some gas liquid as well - and sometimes not.

Funny :)

... But I still think it averages out in the end with cumulative fuel consumption (in a normal car with a single tank). I agree your tank to tank variation can be higher with small fill-ups (and if those figures are your concern - eg for testing purposes - fair enough.)

I still think the long term figure over several to many tanks is reliable, regardless of fill amounts.

MetroMPG

Over many many whole tanks it probably work out better , but not with top ups.
Of course with a MPG computer this is totally irelevant as it doesnt require you to rely on the tank as being as relaible capacity.

I will explain it with another example.

I fill up the car but unknown to me there is say 5 litres of trapped air in the tank which does not get filled with gas.
I drive my hundreds of miles noting the before and after reading.
I then refill the car and take down the litres filled , but this time perhaps due to a differnet parking angle of the car , air is vented from the tank.
I now fill an extra 5 litres into my tank than on the previous occasion.

I calculate the FE figure results ,, and it looks like that I have used 5 litres more gas since my last fill up.

again ,, from full tank to another , its not too big a problem , but if you run this scenerio again but using 1/4 tank FE tests the error could be huge.


On the next FE test it could look like you have used 5 litres less fuel - you cant experiment with so much variation, especially when it looks like I have just saved 5 litres over a quater of a tank.

Of course not all cars would be like this , some wil fill completely ,,but surely some still have this problem.

I wonder if our tanks are very different.

I've never had this problem with my Suzukiclone when filling at the same pump, parked in the same spot - that's the key.

Out of the box, the ScanGauge and the pump were within a couple per cent of each other - and the error was in the ScanGauge, not the fill amount. I've been able to calibrate the SG so that recent fills show only a 0.5% discrepancy.

How many liters is your car's tank? Mine's 40 / 10.5 US gal.

EDIT: my fills are typically between 10 to 20 liters. I think you'd be safe doing this if you used the same pump & parking spot for each fill.

Hi MetroMPG

Ime not sure of the exact size of my tank.
The most I have ever fit is 36 litres into it rite up the tube , so perhaps its 35 litres in size but probably it is 40.

¨I always try to fill at the samp pump, parked in the same spot - that's the key.¨

I agree , it is a must , supossing a tank does have an air trap filling up at the exact same pump (whatever the floor level may be) and addopting the same fill method will get far more consistant results.

My next fill up should give a good baseline indication of what my cars FE is.
As its slightly different in specs to the average metro I have no figures available from others experience., and my Suzuki handbooks FE figure seems very optimistic.

I think I will come in at a lower FE number than what they quote.

Hi theclencher

Yeah , i saw that thread b4., but I dont think it address the exact point that I have raised here.
But even when estimating how far up our filler neck the gas ends at could introduce another variable that would be especially noticable with small fills.

A scribed mark on the inside of the tube might be a good guide as to fuel height instead of just relying on memory.

Might need to reinvent the fuel stick like what was used in old VW's :-)

Hmm, i was just thinking about this, I also have the same problem, but mines even worse because I mix my own fuel, E-42.5, so I never fill it up to the same levels :S. Not to mention my fuel gauge hasn't moved since my last boost tuning run.

Anyways, the problem i see is if you fill up with gas in the morning, it's colder-denser, the pump measures volume, but the engine injects it by mass, (1g of gas does 1g of gas's work.) So if you fill up one day when its cold, and one day when it's hot you could have a noticeable difference in the actual amount of gas your getting!

Yawwwn , I still have about 50 miles left in my tank before i will refill.
At my usage rate thats another 2 , maybe 2 and a half weeks. :-(

Big oil doesnt have much of a grip on me ,, well ,, if I ignore transport costs on the good that I buy that is.