Weird Fuel Consumption

[heavyD]

Alright, the last thing I wanted to do was open up a fuel consumption thread, but I'm really foxed here...

My car usually does 6 - 7 kmpl in traffic with regular driving. And I generally wait until the tank is half full to go fill her up again. So after pumping a full tank, and after running around for a couple of days (about 100 kms), I went to the station again. To my amazement, only around 10 litres were pumped! She had done almost 10 kmpl!

After this incident, and after another 100 kms, I pumped a full tank again and found that she had returned to her normal consumption habits. So I thought that it was some mistake. Maybe the fuel station attendant hadn't pumped a full tank or something.

However, this morning, after another 100 kms, I pumped full tank yet again. And found out that she had done about 10 kmpl! Again!

Admittedly, this was all after a bit of economical driving, but how can this happen? How can keeping the tank at full and filling her up every 100 kms decrease fuel consumption by such a large margin? Or are the fuel station attendants simply not pumping a full tank?

Am I missing something here?

[jaga]

Are u pumping fuel from the same station? By the way what is ur car?

[Uditha88]

Are u pumping fuel from the same station? By the way what is ur car?

Good question!

BTW:

Economy will depend on the:

1. Way u drive

2. Road traffic jam for that particular 100 km

3. Way of u use AC

4. Load u carry.

Note:

Some station guys stops pumping when pump makes a noise. Others keep pumping after that even properly inspecting the tank visually.

This even causes for different pumping capacity. (But this will be very small change though)

[shanX]

And verify the accuracy of your fuel gauge

[heavyD]

Are u pumping fuel from the same station? By the way what is ur car?

The first instance it was the same station. Previous fillups were done there too. However the last two times were at different stations. And it's a Hyundai Tuscani.

Good question!

BTW:

Economy will depend on the:

1. Way u drive

2. Road traffic jam for that particular 100 km

3. Way of u use AC

4. Load u carry.

Note:

Some station guys stops pumping when pump makes a noise. Others keep pumping after that even properly inspecting the tank visually.

This even causes for different pumping capacity. (But this will be very small change though)

Yeah, I took all these factors into consideration. And they've been pretty consistent. So this economy increase only happened when I've traveled about 100 kms and topped up the tank again. If I wait until I've used up about half my tank, I get the usual figures.

True, the uneven amount of fuel the stations guys pump would probably the deciding factor. But it would have to be a difference of about 5 litres or more. And that's a lot, isn't it?

And verify the accuracy of your fuel gauge

But I don't use that to determine fuel consumption.

[Uditha88]

And they've been pretty consistent. So this economy increase only happened when I've traveled about 100 kms and topped up the tank again. If I wait until I've used up about half my tank, I get the usual figures.

But I don't use that to determine fuel consumption.

Hmm.. well. Just as a clue for u, I too have a personal experience that when I fill tank full, my fuel indicator

takes some more time to come down after even drive 50 60 Km.

(but i think this is only a visual mismatch & vehicl has burnt required fuel for run. May b not 100%

accuracy between fuel capacity available n fuel meter reading...)

BTW. Please find below the proper way to measure ur f. economy. It advices us to rung til u get less that 1/2 of ur tank before reading.

1 Go to the gas station and fill up the fuel tank.

2 Record the mileage, before even pulling away from the pump. We will call this Mileage A.

3 Drive normally until the tank is less than half full.

4 Fill up the tank again (preferably at the same station using the same pump as pumps may be calibrated differently). This time, pay attention to how many gallons it takes to fill up the tank. This is usually shown at the pump.

5 Record the mileage again, just like before. We will call this Mileage B.

6 Subtract Mileage A from Mileage B. This will give you the number of miles you drove since your last fill-up.

7 Divide your answer by the number of gallons it took to fill up your tank. This will give you your car's MPG.

[Watchman]

I'd think the varying figures are probably because you pump at very short intervals. Example, how do you define 'full'.. do you fill the tank to the brim? or till the pump cuts off? I'd imagine the piping between cap and tank would hold a good 2 liters or so...

in which case if you fill it to the brim you'd b pumping 2 exctra liters which crew up ur calculations.

example:

if you pump upto the brim you're puming 10 liters and doing 100km... so you get 10kmpl..

but say the next time the guy stops pumping at the cut off.. you might not be filling it to the brim .. hence 2liters less... and now you'll get a figure of 100/8 = 12.5kmpl thats a 25% increase...

why dont you try do the numbers by using about 3/4 of the tank... you'll get a much smaller margin of error...

[heavyD]

Hmm.. well. Just as a clue for u, I too have a personal experience that when I fill tank full, my fuel indicator

takes some more time to come down after even drive 50 60 Km.

(but i think this is only a visual mismatch & vehicl has burnt required fuel for run. May b not 100%

accuracy between fuel capacity available n fuel meter reading...)

Yeah, I think the indicator doesn't come down for a while after a full tank is cause as Dilesh says below there is fuel between the tank and cap. That's what's being burnt and it doesn't show up on the meter. It happens to me too. When I fill up after running a short while, it doesn't even register on the trip computer.

I'd think the varying figures are probably because you pump at very short intervals. Example, how do you define 'full'.. do you fill the tank to the brim? or till the pump cuts off? I'd imagine the piping between cap and tank would hold a good 2 liters or so...

in which case if you fill it to the brim you'd b pumping 2 exctra liters which crew up ur calculations.

example:

if you pump upto the brim you're puming 10 liters and doing 100km... so you get 10kmpl..

but say the next time the guy stops pumping at the cut off.. you might not be filling it to the brim .. hence 2liters less... and now you'll get a figure of 100/8 = 12.5kmpl thats a 25% increase...

why dont you try do the numbers by using about 3/4 of the tank... you'll get a much smaller margin of error...

Well, I've watched as the guys pump the petrol. They keep pumping even after the cut off, so presumably they fill the gap between the tank and cap. But I haven't observed it close up. And what you say makes sense, I should change the way I make this calculation.

[scds]

once I had an old car which fuel indicator needle always behaved queer if the car was not quite horizontal.

I can remember once I pumped diesel in to it thinking the tank was 3/4rds full and it began to overflow

[aamarakoon]

It's probably maths.

When you consider the possible variation of amount of petrol pumped at a given time, any error will be more obvious when calculated for, say 3/4 tank than 1/2 or 1/4.

This happens to me too!

[L board]

Please ask the your car to behave in a better manner

Lol