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

Mildly_Miffed

s71hj said:

With the 30 year old zafira, full to almost empty I just got 34.7 mpg. . A few days ago getting it down to only around half a tank full before filling up i got 44.71 mpg.

I know basic numeracy is unfashionable these days, but just pause and think how realistic that is.

Full tank = 34.7mpg

Top tank half = 44.7mpg. To average 34.7 from full tank, the second half of the tank would have to return 24.7mpg

Does that seem even SLIGHTLY feasible to you?

s71hj

Mildly_Miffed said:

s71hj said:

With the 30 year old zafira, full to almost empty I just got 34.7 mpg. . A few days ago getting it down to only around half a tank full before filling up i got 44.71 mpg.

I know basic numeracy is unfashionable these days, but just pause and think how realistic that is.

Full tank = 34.7mpg

Top tank half = 44.7mpg. To average 34.7 from full tank, the second half of the tank would have to return 24.7mpg

Does that seem even SLIGHTLY feasible to you?

Is it really necessary to phrase it in quite that way! That's what I'm getting when I feed it into an mpg calculator 

CliveOfIndia

s71hj said:

With the 30 year old zafira, full to almost empty I just got 34.7 mpg. . A few days ago getting it down to only around half a tank full before filling up i got 44.71 mpg.

So you're talking about 2 journeys on different days.  Was the journey exactly the same - same route, same amount of traffic, same speed, same amount of people/luggage in the car?  Was the weather the same - same temperature, same amount (or lack of) rainfall?  Did you have the windows either open or closed for the same amount of time on each journey?  Did you use the aircon the same amount on each journey?  All of these things will make a difference - and a far more significant difference than a few kilograms of fuel.

Mildly_Miffed

s71hj said:

Mildly_Miffed said:

s71hj said:

With the 30 year old zafira, full to almost empty I just got 34.7 mpg. . A few days ago getting it down to only around half a tank full before filling up i got 44.71 mpg.

I know basic numeracy is unfashionable these days, but just pause and think how realistic that is.

Full tank = 34.7mpg

Top tank half = 44.7mpg. To average 34.7 from full tank, the second half of the tank would have to return 24.7mpg

Does that seem even SLIGHTLY feasible to you?

Is it really necessary to phrase it in quite that way! That's what I'm getting when I feed it into an mpg calculator 

I'm sorry if you don't like the way I phrased it, but the point stands.

It quite simply does not pass even the most basic of sniff tests, which really should be blatantly obvious.

BTW, why do you need an "MPG calculator"?
You need to know two variables and one constant.
Fuel used. Miles travelled. 4.5 litres in a gallon. That's it.

Stubod

I keep an "actual" record of my fuel consumption, (brim to brim), and it is surprising what the difference in temperature makes. In the colder / Winter months my average is 5 mpg lower than the summer months....

s71hj

Mildly_Miffed said:

s71hj said:

Mildly_Miffed said:

s71hj said:

With the 30 year old zafira, full to almost empty I just got 34.7 mpg. . A few days ago getting it down to only around half a tank full before filling up i got 44.71 mpg.

I know basic numeracy is unfashionable these days, but just pause and think how realistic that is.

Full tank = 34.7mpg

Top tank half = 44.7mpg. To average 34.7 from full tank, the second half of the tank would have to return 24.7mpg

Does that seem even SLIGHTLY feasible to you?

Is it really necessary to phrase it in quite that way! That's what I'm getting when I feed it into an mpg calculator 

I'm sorry if you don't like the way I phrased it, but the point stands.

It quite simply does not pass even the most basic of sniff tests, which really should be blatantly obvious.

BTW, why do you need an "MPG calculator"?
You need to know two variables and one constant.
Fuel used. Miles travelled. 4.5 litres in a gallon. That's it.

I fed the figures into an mpg calculator in the light of your impolite, in fact downright rude comment about "basic numeracy".

Mildly_Miffed

s71hj said:

Mildly_Miffed said:

s71hj said:

Mildly_Miffed said:

s71hj said:

With the 30 year old zafira, full to almost empty I just got 34.7 mpg. . A few days ago getting it down to only around half a tank full before filling up i got 44.71 mpg.

I know basic numeracy is unfashionable these days, but just pause and think how realistic that is.

Full tank = 34.7mpg

Top tank half = 44.7mpg. To average 34.7 from full tank, the second half of the tank would have to return 24.7mpg

Does that seem even SLIGHTLY feasible to you?

Is it really necessary to phrase it in quite that way! That's what I'm getting when I feed it into an mpg calculator 

I'm sorry if you don't like the way I phrased it, but the point stands.

It quite simply does not pass even the most basic of sniff tests, which really should be blatantly obvious.

BTW, why do you need an "MPG calculator"?
You need to know two variables and one constant.
Fuel used. Miles travelled. 4.5 litres in a gallon. That's it.

I fed the figures into an mpg calculator in the light of your impolite, in fact downright rude comment about "basic numeracy".

Again, I apologise if you don't like the way I phrased it, but it should only take a very quick glance to show you how the tank average is going to have to change over the bottom half in order to go from the top half's average to the whole tank's average.

If that alone didn't ring alarm bells in your internal sense checker, then...

s71hj

CliveOfIndia said:

s71hj said:

With the 30 year old zafira, full to almost empty I just got 34.7 mpg. . A few days ago getting it down to only around half a tank full before filling up i got 44.71 mpg.

So you're talking about 2 journeys on different days.  Was the journey exactly the same - same route, same amount of traffic, same speed, same amount of people/luggage in the car?  Was the weather the same - same temperature, same amount (or lack of) rainfall?  Did you have the windows either open or closed for the same amount of time on each journey?  Did you use the aircon the same amount on each journey?  All of these things will make a difference - and a far more significant difference than a few kilograms of fuel.

Sure, different journeys. But this full to nearly empty versus full to half fullish discrepancy has seemed to apply across a number of calculations on similar days with broadly similar other variables- aircon permanently off windows never open similar mix of urban stop start and dual carriageway /steady 50 to 60ish speed with full to half empty always giving out markedly better fuel efficiency.

Mildly_Miffed

s71hj said:

...with full to half empty always giving out markedly better fuel efficiency.

85% better from the top half to the bottom half, apparently.

s71hj

This
https://www.fuelgenie.co.uk/news/is-it-more-economical-to-fill-up-your-tank-a-closer-look-at-fuel-economy/
 suggests fuel tank condensation may be a factor but I have no idea from a mechanical or technical perspective if this makes any sense!!!