Fuel efficiency and Older cars : getting tank to empty before filling or not.

Grey_Critic

I seem to recall reading that a fuel gauge is only an indication and the manufacturer does not warrant the accuracy.

eskbanker

Can't believe we're still talking about fuel gauges on page 7, long after OP clarified that they play no role in their calculations!

To restate what OP is saying:

• they fill to the brim each time
• the calibrated pump obviously shows the exact number of litres being dispensed
• they record the number of miles in between each fuelling
• this gives a simple conversion to an mpg figure in between consecutive fills
• this mpg figure when filling with, say, 20 litres is better than when filling with, say, 50 litres

The total tank capacity, and the fraction of it in use, aren't actually part of this process.

paul_c123

If the OP really is saying that, he hasn't provided the figures (raw data) requested twice now, so someone else can check his maths.

s71hj

paul_c123 said:

s71hj said:

paul_c123 said:

So you're saying, you can do more miles if you use 2/3 of the tank, than you do if you use 1/3 of the tank?

Not more miles, more miles per gallon if i use half a tank or less from full compared with if I use pretty much a whole tank from full.

But you're using more than half the tank - that's the whole point. The mpg is roughly the same in both halves of the tank, but the gauge is displaying "half" when its actually a little less than half (or, your "empty" isn't really empty, so the 2nd half isn't quite as much as you think it is).

Put simply: your fuel gauge isn't being interpreted properly.

You still haven't posted your raw data which we can use to check your assumptions and calculations.

I'm basing nothing on what my fuel gauge looks like!!!!

s71hj

eskbanker said:

Can't believe we're still talking about fuel gauges on page 7, long after OP clarified that they play no role in their calculations!

To restate what OP is saying:

• they fill to the brim each time
• the calibrated pump obviously shows the exact number of litres being dispensed
• they record the number of miles in between each fuelling
• this gives a simple conversion to an mpg figure in between consecutive fills
• this mpg figure when filling with, say, 20 litres is better than when filling with, say, 50 litres

The total tank capacity, and the fraction of it in use, aren't actually part of this process.

Thank you!!!

paul_c123

raw data  

s71hj

paul_c123 said:

raw data  

I may at some point if I find the notes from the various times I've done this but frankly given the seemingly determined inability to actually understand what I'm saying despite repeated clarifications and the patronising manner, no matter how many times O try to politely clarify, there seems little point. I mean why would I be repeatedly getting the maths wrong but ONLY for half tank fills?

Grey_Critic

Fuel guages are only an indicator - You cannot actually rely on filling a tank to the exact same level each time. The only accurate method would be to use a flow meter and an accurate odometer and then do the maths but even then iot would vary depending upon the various other factors. 

A rolling road or a closed circuit will produce different figures to the open road. I once was asked to be an observer for a test when a customer tried to make a claim that his consumption of fuel was excessive. The car was wired up with flow meter etc and the customer was invited to drive - all was recorded and only then could he understand all the factors involved.  Stop/start heavy and light traffic, Up and down hills and changing  road conditions.

On Tuesday I had a hefty bill to pay for some repairs. First time for years as my mate who has looked after my cars for years has retired - basically good or bad mpg etc is one of the joys of motoring

Stubod

Grey_Critic said:

Fuel guages are only an indicator - You cannot actually rely on filling a tank to the exact same level each time. The only accurate method would be to use a flow meter and an accurate odometer and then do the maths but even then iot would vary depending upon the various other factors. 

A rolling road or a closed circuit will produce different figures to the open road. I once was asked to be an observer for a test when a customer tried to make a claim that his consumption of fuel was excessive. The car was wired up with flow meter etc and the customer was invited to drive - all was recorded and only then could he understand all the factors involved.  Stop/start heavy and light traffic, Up and down hills and changing  road conditions.

On Tuesday I had a hefty bill to pay for some repairs. First time for years as my mate who has looked after my cars for years has retired - basically good or bad mpg etc is one of the joys of motoring

...as already stated, the fuel gauge is not a factor in this. The OP fills the tank to the "brim" each time they fill up...

s71hj

Stubod said:

Grey_Critic said:

Fuel guages are only an indicator - You cannot actually rely on filling a tank to the exact same level each time. The only accurate method would be to use a flow meter and an accurate odometer and then do the maths but even then iot would vary depending upon the various other factors. 

A rolling road or a closed circuit will produce different figures to the open road. I once was asked to be an observer for a test when a customer tried to make a claim that his consumption of fuel was excessive. The car was wired up with flow meter etc and the customer was invited to drive - all was recorded and only then could he understand all the factors involved.  Stop/start heavy and light traffic, Up and down hills and changing  road conditions.

On Tuesday I had a hefty bill to pay for some repairs. First time for years as my mate who has looked after my cars for years has retired - basically good or bad mpg etc is one of the joys of motoring

...as already stated, the fuel gauge is not a factor in this. The OP fills the tank to the "brim" each time they fill up...

Thanks....... again!