GTTDI economy Drive Vic/SA

[STV4SYT]

My eyes never left the road

[POLARBEAR666]

I have found that if you look at the AVerage fuel economy immediately after filling up .. It should be almost dead accurate to the actual fuel use (L filled up vs km on the trip meter since last refuelling). I think the computer is doing the same accurate calculation when it senses the car has fuelled up again. FYI my TSI can get down to 5.8L.

[jb747]

The wind will have a very large effect. The aerodynamic drag rises as the square of the speed, so any increase will come in pretty large increments.

[POLARBEAR666]

It is only exact IF you fill to precisely the same point every time. If you haven't got a mark on the filler tube then it is very unlikely you always fill to the exact same point every time.

I know that with the usual diesel froth and occassional filler pipe "burps" and varying levels of patience when I'm filling up my TDI that I can sometimes put anything up to 2 litres more or less each time I fill. That is why I said it is better to average over 2 or 3 fills to eliminate over or under fills.

If you ALWAYS take 10 minutes or so to let the froth settle and keep putting in a little bit at a time until you see clear (froth free) diesel at the top of your filler pipe then you will be getting an exact figure, if you don't then I very much doubt it. I know I'm more patient than most people, and I'm not that patient Plus the last time I tried to, the auto shut-off wasn't quite sensitive enough and I got diesel all down the side of my van - I'm not doing that again

Actually your wrong. It is EXACT! because you are taking the fuel filled off the fuel pump's readout. Not anything to do with the cars readout of fuel.

The second part of the equation is your trip meter which you reset before starting the car after each fill.

Exact fuel consumption is the result. I have also checked my speedo against gps speed and its within 1km which would mean the cars measurements of distance for the trip meter would be pretty accurate.

[VW Convert]

Actually your wrong. It is EXACT! because you are taking the fuel filled off the fuel pump's readout. Not anything to do with the cars readout of fuel.

The second part of the equation is your trip meter which you reset before starting the car after each fill.

Exact fuel consumption is the result. I have also checked my speedo against gps speed and its within 1km which would mean the cars measurements of distance for the trip meter would be pretty accurate.

Sorry this is not correct, to be accurate you must fill to the same point as the tank was when you reset the trip meter. It has nothing to do with the cars readout of fuel, just the actual amount of fuel in the tank. What you are measuring is the amount of fuel you used for the journey, to accurately ascertain that you have to fill to the same point.

Cheers

George

[gregozedobe]

Actually your wrong. It is EXACT! because you are taking the fuel filled off the fuel pump's readout. Not anything to do with the cars readout of fuel.

The second part of the equation is your trip meter which you reset before starting the car after each fill.

Exact fuel consumption is the result. I have also checked my speedo against gps speed and its within 1km which would mean the cars measurements of distance for the trip meter would be pretty accurate.

Sorry PBdevil,

You still haven't understood how to work out an exact figure for fuel consumption. I agree that you are getting an exact measurement of how much fuel you put into your tank, but that has only an approximate relationship with how much fuel your car actually used to travel that distance.

If after a trip I underfill the tank by 3 litres, then I will appear to have lower consumption for that trip (compared with what the vehicle actually used). If on the next trip I then fill the tank right to the brim of the filler pipe this trip will seem to have higher consumption when it may have been exactly the same. It has nothing to do with the fuel gauge or the MFD's calculated consumption. Because of this potential variablity in filling your tank I always advise to average your consumption figures over at least 2-3 fills.

I agree the odo and trip meters seem pretty accurate for the distance travelled, but that is not the area where you are confused.

Perhaps I can give a theoretical example, and by taking it to an illogical extreme I can show up the flaw in your reasoning ?

Warning, long and involved explanation follows !


I happen to have two identical cars and two identical drivers that get exactly the same fuel economy (unlikely, but very useful for the purposes of my explanation). Both cars were filled up right to the brim and the trip meters were both set to zero before starting Trip 1.

-------------------- TRIP 1 ---------------------------------------------

Both cars were then driven 600Kms. Both engines actually used 36 litres of fuel to travel this distance (I have fitted both cars with a device to accurately measure actual fuel consumption, so I know this with certainty).

Car A was filled up by an impatient service station attendant who only managed to get 32 litres into the tank before giving up. In fact the tank is now only part full, and would actually take 4 more litres if filled completely.

Car B was filled up by the very patient driver, who manages to put 36 litres into it (thus filling the tank completely).

Both cars actually used 6 litres/100km ( 36/6 ), but according to your "exact" method of calculating consumption, Car A used 5.3 litres/100km ( 32/6 ), and Car B used 6 litres/100km ( 36/6 ) for trip 1.

Now I will send these two cars on a second trip:

-------------------- TRIP 2 ---------------------------------------------

Both cars were then driven a further 800Kms. Both engines actually used 48 litres of fuel to travel this distance (remember I have fitted both cars with a device to accurately measure actual fuel consumption). So again, both cars actually used 6 litres/100Km.

This time I got the patient driver to fill both tanks, thus ensuring both tanks were completely full. This means we put 52 litres into Car A (it was underfilled by 4 litres at the previous fill up, so 4 + 48 =52 ), but of course only fitted 48 litres into Car B.

By your "exact" calculation method, this time Car A used 6.5 litres/100km ( 52/8 ) for trip 2, while Car B was 6 litres/100km ( 48/8 ).

-----------------------------------------------------------------------

Can you now see where the problem lies with your "exact" calculation method ?

Using my "average" method I get an average of 6 litres/100km for both cars for the total of Trip 1 + Trip 2. The calculation is: Car A ( 32 + 52 ) /( 6+8 ), and Car B ( 36 + 48 ) /( 6+8 ).

I hope that clarifies it for you.

[POLARBEAR666]

Man this is amazing.

If you fill the car till it does 1 click every time then it will be exact. Obviously if you don't fill the car full each time you can't tell how much fuel you used. I never assumed anyone would be stupid enough to try my method when they had not filled the car the same way each time.

I fill till 1 click. Reset trip meter to zero. Drive X km. Then when I refill I note down the L filled on the petrol bowser to get it back up to 1 click and that is how much fuel you used to travel the Km on the trip meter.

Its pretty simple!

[gregozedobe]

If you fill the car till it does 1 click every time then it will be exact.

OK, I tried (actually I tried very hard), but now I give up !!!!!!!!!!!!!

[POLARBEAR666]

OK, I tried (actually I tried very hard), but now I give up !!!!!!!!!!!!!

Are you suggesting that if you fill till the first click it will be vastly different fuel quantities each time? 4L different!!???

[gregozedobe]

Are you suggesting that if you fill till the first click it will be vastly different fuel quantities each time? 4L different!!???

I'm not just suggesting it, I'm saying it happens to me on a regular basis (not absolutely every time, but often enough that I wouldn't even consider stopping filling after the first click). When filling my TDI at different pumps, after the "first click" I can sometimes put an extra 5 or 6 litres in, other times I can only put in another 2 or 3 litres, so YES, I would get results differing by 4 litres if I followed your method. By my standards that is a long, long way from being "exact"

I'm wondering if you have had much experience with the large amount of froth you get when filling with diesel ? This thread is about diesel fuel economy (not petrol). The problem with diesel froth is bad enough that BP have chosen to use an "anti-froth" additive in their new "BP Ultimate diesel".

[POLARBEAR666]

I'm going to run a tank of diesel and see if it is different.

[gregozedobe]

I'm going to run a tank of diesel and see if it is different.

It WILL be different alright, in oh so many ways

I know putting petrol into a diesel engine is very bad, I'm not so sure what your 1.4 supercharged and turbocharged petrol motor will make of its batch of new, oily fuel

[STV4SYT]

It WILL be different alright, in oh so many ways

I know putting petrol into a diesel engine is very bad, I'm not so sure what your 1.4 supercharged and turbocharged petrol motor will make of its batch of new, oily fuel

I have run a small amount of petrol in my Golf, servo down at rye had got a black (diesel)pump handle on the premium unleaded.

Luckily i only had put in about 1.5 litres so and could just use it as some extra lubrication.

[Mr X]

Just calculated mine over 9700 Km, 5.8 l/100km.

That's an average so there will be swing and roundabouts.

Given the total litres +/- 1 or 2 is not going to significantly influence it.

Est percentage 67% city 33% country

I am very happy

Calculations from actual litres used vs kilometres

MFD indicates 6,3 l/100

Mr X

[gerhard]

Everyone is assuming the metered amount at the servo is accurate, in my experience servo owners will readily short sell.

So all the arguing about decimal point accuracy and filling technique is futile.

Who really gives a rats if the economy is 5.97623l/100 - isn't 5.8 or 5.9 close enough?