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

Goudy

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.

Yes, I referred to this earlier.

The OP is getting different MPG averages. 
One with 20 litres being used and one with 50 litres being used.

The only variable in these calculations is the miles/time covered is much greater with 50 litres.
They aren't like for like calculations so will never produce a comparable result.

The OP obviously covers more miles/time and driving variation with the calculation based on 50 litres.
This could include more cold starts and warm ups in the mornings, more idling at junctions and traffic lights, more hold ups in traffic etc than the shorter, 20 litre test.


My car tells me the current calculation of average MPG and it's based on the distance in miles since the last time I reset it. It's around 40 mpg at the moment but I reset it around 800 miles ago.

If I reset it now and drive 300 miles, it probably will never match 40 mpg. 
Even though I think my trips are similar, I've done less cold starts, been held up less, idled less, stopped at the lights less and so on.
 
The difference in the calculation between the two is 500 miles and that will alter the result.
If I carried on and left it for another 500 miles, I'm certain I will get to the 40 mpg mark again as the variables will all be similar.

s71hj

Goudy said:

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.

Yes, I referred to this earlier.

The OP is getting different MPG averages. 
One with 20 litres being used and one with 50 litres being used.

The only variable in these calculations is the miles/time covered is much greater with 50 litres.
They aren't like for like calculations so will never produce a comparable result.

The OP obviously covers more miles/time and driving variation with the calculation based on 50 litres.
This could include more cold starts and warm ups in the mornings, more idling at junctions and traffic lights, more hold ups in traffic etc than the shorter, 20 litre test.


My car tells me the current calculation of average MPG and it's based on the distance in miles since the last time I reset it. It's around 40 mpg at the moment but I reset it around 800 miles ago.

If I reset it now and drive 300 miles, it probably will never match 40 mpg. 
Even though I think my trips are similar, I've done less cold starts, been held up less, idled less, stopped at the lights less and so on.
 
The difference in the calculation between the two is 500 miles and that will alter the result.
If I carried on and left it for another 500 miles, I'm certain I will get to the 40 mpg mark again as the variables will all be similar.

Sure, but in that sense, no mpg calculations are like for like with different numbers of cold starts, different time at lights, variable prevailing winds etc. Over multiple journeys however these things average out.

MeteredOut

s71hj said:

Goudy said:

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.

Yes, I referred to this earlier.

The OP is getting different MPG averages. 
One with 20 litres being used and one with 50 litres being used.

The only variable in these calculations is the miles/time covered is much greater with 50 litres.
They aren't like for like calculations so will never produce a comparable result.

The OP obviously covers more miles/time and driving variation with the calculation based on 50 litres.
This could include more cold starts and warm ups in the mornings, more idling at junctions and traffic lights, more hold ups in traffic etc than the shorter, 20 litre test.


My car tells me the current calculation of average MPG and it's based on the distance in miles since the last time I reset it. It's around 40 mpg at the moment but I reset it around 800 miles ago.

If I reset it now and drive 300 miles, it probably will never match 40 mpg. 
Even though I think my trips are similar, I've done less cold starts, been held up less, idled less, stopped at the lights less and so on.
 
The difference in the calculation between the two is 500 miles and that will alter the result.
If I carried on and left it for another 500 miles, I'm certain I will get to the 40 mpg mark again as the variables will all be similar.

Sure, but in that sense, no mpg calculations are like for like with different numbers of cold starts, different time at lights, variable prevailing winds etc. Over multiple journeys however these things average out.

I'd suggest, unless your car somehow defies the laws of physics, that was not the case when the MPG went from 33 to 40 with a fuller tank.

Goudy

s71hj said:

Goudy said:

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.

Yes, I referred to this earlier.

The OP is getting different MPG averages. 
One with 20 litres being used and one with 50 litres being used.

The only variable in these calculations is the miles/time covered is much greater with 50 litres.
They aren't like for like calculations so will never produce a comparable result.

The OP obviously covers more miles/time and driving variation with the calculation based on 50 litres.
This could include more cold starts and warm ups in the mornings, more idling at junctions and traffic lights, more hold ups in traffic etc than the shorter, 20 litre test.


My car tells me the current calculation of average MPG and it's based on the distance in miles since the last time I reset it. It's around 40 mpg at the moment but I reset it around 800 miles ago.

If I reset it now and drive 300 miles, it probably will never match 40 mpg. 
Even though I think my trips are similar, I've done less cold starts, been held up less, idled less, stopped at the lights less and so on.
 
The difference in the calculation between the two is 500 miles and that will alter the result.
If I carried on and left it for another 500 miles, I'm certain I will get to the 40 mpg mark again as the variables will all be similar.

Sure, but in that sense, no mpg calculations are like for like with different numbers of cold starts, different time at lights, variable prevailing winds etc. Over multiple journeys however these things average out.

Yes that's why there are standards for testing, then you can repeat those standards and compare.

I reckon if you set the more random variable in your calculation to one more static, you would get a more reliable MPG figure and both would likely closely match each other.

Say drive to the motorway, full up and drive 100 or so miles at a set speed, after which refill and work out your MPG from those figures.
Then carry on another 300 or so miles at the same set speed, then refill and do another calculation on those figures.

But remember, it's no use drafting an Artic on one test and not the other, you'll just spoil the results!

eskbanker

Goudy said:

But remember, it's no use drafting an Arctic on one test and not the other, you'll just spoil the results!

That would certainly distort any comparison by giving significant variances in ambient temperature....

prowla

I don't think mpg can be viewed as an absolute measure; there will be variances according to:

• The weight of the car: how many people are in it, goods it is carrying, etc.
• Whether it is towing anything (as pre previous comments).
• Traffic conditions, start-stop, roadworks, traffic lights.
• Type of journey: short runs to the shop, long motorway runs.
• Whether you have the heating or a/c on.
• Whether it has a sport/normal/economy mode.
• How heavy your foot is on the pedals (on the given journey).
• Does it have a roof box.
• Weather conditions: wind, rain, snow, ice.
• Whether you are making special journeys or detours to fill up.
• etc.

My opinion is that any of the above would likely have a greater impact than how full the fuel tank happens to be.

Stubod

In general I maintain the same basic driving style do similar journeys and I get an overall variation over the last 3 years of around 10mpg. Most of these seems to be down to temperature, the lower figure being during winter driving, and the higher figure during the summer, (NB self charge hybrid).

prowla

Ah - the self-charge hybrid also adds a variable.

HedgehogRulez

How can you be certain you’ve hit half way in your tank? 

droopsnoot

HedgehogRulez said:

How can you be certain you’ve hit half way in your tank? 

The OP has said several times that they're working it out based on how much fuel they have to put back in the tank until the filler "clicks", and how many miles they've done in that time.