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

Sarahspangles

s71hj said:

I've always worked on the assumption that getting the fuel tank down to near empty is best due to the weight of the fuel decreasing mpg.

My mechanic says to fill up when the tank is quarter full, because there may be crud settled at the bottom of the tank and this will block nozzles if it’s used. I have this mental image of the sediment at the bottom of a bottle of port….. He may be saying this because he’s in the top three I would ring if I ran out of petrol!

born_again

s71hj said:

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.

Is your gauge showing half tank. Exactly 50% of the capacity?

Many cars are no where near.

Not sure how you can say "I brim to brim measure". Than say that you get better MPG when tank is half empty?
As brim to brim is exactly what it says. You get the MPG from the amount of fuel used by filling up to the same point.
As no way can you know, unless filling all the way up, that you are at the same level?

Sounds more like the 2nd trip had a following wind & maybe more downhill?

s71hj

born_again said:

s71hj said:

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.

Is your gauge showing half tank. Exactly 50% of the capacity?

Many cars are no where near.

Not sure how you can say "I brim to brim measure". Than say that you get better MPG when tank is half empty?
As brim to brim is exactly what it says. You get the MPG from the amount of fuel used by filling up to the same point.
As no way can you know, unless filling all the way up, that you are at the same level?

Sounds more like the 2nd trip had a following wind & maybe more downhill?

Fill up to the brim, fill back up to the brim 

WellKnownSid

s71hj said:

born_again said:

s71hj said:

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.

Is your gauge showing half tank. Exactly 50% of the capacity?

Many cars are no where near.

Not sure how you can say "I brim to brim measure". Than say that you get better MPG when tank is half empty?
As brim to brim is exactly what it says. You get the MPG from the amount of fuel used by filling up to the same point.
As no way can you know, unless filling all the way up, that you are at the same level?

Sounds more like the 2nd trip had a following wind & maybe more downhill?

Fill up to the brim, fill back up to the brim 

I remember on another forum about two years ago there was a long standing member who had just returned from holiday in their small SUV.

He was delighted to inform everyone that - despite never seeing more than 38 mpg or thereabouts around town - he’d travelled down one side of Scotland (the downhill part?) and due to his careful driving had averaged a steady 85 mpg on his careful brim to brim calculation.

There followed pages of debate - mostly denial - but a few supporting posts. Particularly fresh fuel, the dew point that morning having set off at 5am, the particular coast road encouraging the use of optimal gear ratios.

The answer of course was much simpler when the missing fuel receipt was eventually found and a more reasonable 41 mpg was posted.

So 100% it’s the maths. 

Grey_Critic

How often do you fill your tank? How many miles do you do between fills? Whilst in theory less weight means better MPG why not strike a balance? Once upon a time when fuel prices were less volatile and cash tighter many of us added say £20 of fuel each week and that was enough for the mileage we did. 

If we went off somewhere say to the seaside for a day then we filled to the brim. but otherwise as long as the needle was above Red then it was sufficient. Only fill to the brim when you actually need to do so that way you get the benefit of both scenarios

Mildly_Miffed

Sarahspangles said:

s71hj said:

I've always worked on the assumption that getting the fuel tank down to near empty is best due to the weight of the fuel decreasing mpg.

My mechanic says to fill up when the tank is quarter full, because there may be crud settled at the bottom of the tank and this will block nozzles if it’s used. I have this mental image of the sediment at the bottom of a bottle of port….. He may be saying this because he’s in the top three I would ring if I ran out of petrol!

Some mechanic, if he's never seen inside a fuel tank.

The pickup does not float on the top.

It sits at the bottom of the tank all the time. If there was sediment there, it'd be sucking it even when the tank was full.

The only thing floating is the level sender for the gauge.

Goudy

s71hj said:

I've always worked on the assumption that getting the fuel tank down to near empty is best due to the weight of the fuel decreasing mpg. However I recently got my 15 year old zafira down to only about 2/3 full and and refilled and the fuel efficiency was about 40 mpg compared to about 33 mpg when getting it near empty. A little Internet research suggested this can be a 'Thing' . Something to do with tank expansion / cooling in older cars. Anyone else have any experience around this? We have a 15 and a 30 year old zafira.

There is a problem with these sorts of calculations.
What actually is two thirds or quarter full in your calculations and where do you get that from, the fuel gauge?

My fuel gauge can hang on full, half full, quarter full for many miles before dropping down.
I guess 50 or so miles, maybe more on the motorway before it drops. 

If I filled up full and ran it until half full on the gauge, I couldn't possible know what is exactly half full and do that again the next time to get a reliable MPG figure as the half a tank mark is so wide, mileage wise.

Also, you don't "use" all of the tank on the gauge.
When it shows completely empty, there is often 5 or 6 litres still in the tank.

The gauge will be calibrated to try and convince you to fill up way before you actually run out.
"Empty" on the gauge doesn't always mean it's actually empty, just a dire warning it soon will be.

Some cars fuel gauges are calibrated so the first half of the tank is actually bigger than the second half, as that second half includes those 5 or 6 litres.

I once proved this much to the chargrin of my late father in his car.
Drove around 40 miles with a completely "empty" tank and 0 miles range on the readout with him panicking.
Even filling it up to the neck later I could only get 56 odd litres in a 60 litre tank.

s71hj

Goudy said:

s71hj said:

I've always worked on the assumption that getting the fuel tank down to near empty is best due to the weight of the fuel decreasing mpg. However I recently got my 15 year old zafira down to only about 2/3 full and and refilled and the fuel efficiency was about 40 mpg compared to about 33 mpg when getting it near empty. A little Internet research suggested this can be a 'Thing' . Something to do with tank expansion / cooling in older cars. Anyone else have any experience around this? We have a 15 and a 30 year old zafira.

There is a problem with these sorts of calculations.
What actually is two thirds or quarter full in your calculations and where do you get that from, the fuel gauge?

My fuel gauge can hang on full, half full, quarter full for many miles before dropping down.
I guess 50 or so miles, maybe more on the motorway before it drops. 

If I filled up full and ran it until half full on the gauge, I couldn't possible know what is exactly half full and do that again the next time to get a reliable MPG figure as the half a tank mark is so wide, mileage wise.

Also, you don't "use" all of the tank on the gauge.
When it shows completely empty, there is often 5 or 6 litres still in the tank.

The gauge will be calibrated to try and convince you to fill up way before you actually run out.
"Empty" on the gauge doesn't always mean it's actually empty, just a dire warning it soon will be.

Some cars fuel gauges are calibrated so the first half of the tank is actually bigger than the second half, as that second half includes those 5 or 6 litres.

I once proved this much to the chargrin of my late father in his car.
Drove around 40 miles with a completely "empty" tank and 0 miles range on the readout with him panicking.
Even filling it up to the neck later I could only get 56 odd litres in a 60 litre tank.

I fill up, note the mileage then fill up again a few days later and note how many miles I've done so the where my fuel gauge is isn't coming into the sum.

rdr

I have had two Zafiras, both 1.8 petrol automatics, the first was a V Reg, the second 09. The petrol gauges were nowhere near linear, in the first one nearly all the fuel went in the first half of the gauge. On occasion we set off half full on the gauge, running out within 20 miles. on the second about three quarters of the fuel was in the first half according to the gauge.
My first ever car, a Fiat Strada, had the fuel tank right at the back and was faster and more economical on a full tank, I suspect because the weight pushed the tail down a little and improved the aerodynamics. you got the same effect by putting stuff in the boot as well.
I always fill to the top when visiting the supermarket with the cheapest fuel available. I argue that any savings due to slightly reduced weight from apart tank cancelled out by having to drive to the fuel station more often and of course the extra time you spend filling up.

maxmycardagain

I use "Spirit Monitor" and only ever "fill to fill", recording mileages, part filling means more stop/start and going off route for fuel

 https://www.spritmonitor.de/en/