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

s71hj

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.

On-the-coast

How could this possibly be a "thing"? (how would an expanded or contracted fuel tank affect your fuel economy?).   BTW garage fuel pumps are calibrated to account for variations in fuel temperature anyway.
Unless perhaps you're talking about the crude instaneous fuel use gauges in your car - in which case just remember that's not particularly accurate.
Bottom line: Less weight carried = less fuel used.
running your tank as near empty as possible (and only refilling it to for example quarter full) would add to your fuel economy, but would marginally increase the dirt heading into your fuel filters and injection system.

flaneurs_lobster

Any chance of posting the links to these internet sources?

Sounds a bit like those suggestions that light-coloured cars are more efficient than dark-coloured identical models (or was it the other way round?).

Stubod

Yes, the rules of physics apply, so the less weight you carry the less fuel you will use. However I can't believe it would make that much difference. 

I remember some motoring programme many years ago that stripped everything none essential out of a car, (including spare wheel), than ran some mpg tests, and there was no discernable difference.

Probably more important to ensure you are maintaining things like tyre pressures.

s71hj

On-the-coast said:

How could this possibly be a "thing"? (how would an expanded or contracted fuel tank affect your fuel economy?).   BTW garage fuel pumps are calibrated to account for variations in fuel temperature anyway.
Unless perhaps you're talking about the crude instaneous fuel use gauges in your car - in which case just remember that's not particularly accurate.
Bottom line: Less weight carried = less fuel used.
running your tank as near empty as possible (and only refilling it to for example quarter full) would add to your fuel economy, but would marginally increase the dirt heading into your fuel filters and injection system.

I have no idea,  I'm not mechanically minded but did a calculation of fuel efficency and there was a striking increase in efficiency filling at 2/3 usage compared to near empty. When I looked it up online there were some references to this phenomena in older cars (but the internet says all sorts of outlandish things) but beyond that I did not follow the reasoning. The fact of the matter though is that all other things were equal ie there was no differential in other factors eg same appx mix of urban and dual carriageway driving driving style tyre pressures. Maybe it was my maths!!!!!

CliveOfIndia

Stubod said:

Yes, the rules of physics apply, so the less weight you carry the less fuel you will use. However I can't believe it would make that much difference. 

I remember some motoring programme many years ago that stripped everything none essential out of a car, (including spare wheel), than ran some mpg tests, and there was no discernable difference.

Probably more important to ensure you are maintain things like tyre pressures.

This is absolutely spot-on.  Yes, reducing weight will improve fuel economy, that's just simple physics.  And it's one reason why newer cars generally don't have a spare wheel as standard.  However, the difference between a full and almost-empty tank of fuel will make such a tiny difference as to be insignificant - you'd need to have some very accurate measuring equipment and test it under strictly-controlled "laboratory" conditions to be able to even measure it.  The weight of a full tank of fuel is a tiny fraction of the overall weight of the car.

In practical terms, as you rightly point out, making sure your tyres are at the correct pressure, making sure you accelerate gently, don't waste energy by braking unnecessarily, things like that, will make a much more noticeable difference to economy.

s71hj said:

the fuel efficiency was about 40 mpg compared to about 33 mpg when getting it near empty.

If you're going by the on-board computer then they are not accurate anyway.  They give you a reasonable estimate, and will give a decent average over the course of several hundred miles, but they're far from "spot-on".

There are so many things that affect fuel economy - tyre pressure, driving style, environment (stop-start town driving gives worse economy than gentle motorway cruising), how much electrical equipment (heated seats, heated windscreen, headlights, etc.) is being used.  Heck, even the air temperature makes a very tiny difference if you really want to get down to the nitty-gritty.

Yes, the additional weight of a tank of fuel will make a very very tiny difference, but it wouldn't come close to accounting for the difference between 33 and 40 mpg.

Mildly_Miffed

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.

Let's just pause and think about this...

Your 2010 Zafira is 1430 to 1599kg kerb weight, depending on spec/engine/etc.

It has a 58L tank.
Diesel's density is 850g/litre, petrol is about 800g/litre.

So, assuming you have a 1430kg diesel engined version, a full tank weighs 49kg. That's just 3.4% of the weight of the car...
If it was the opposite end of the spectrum, 1599kg petrol engined, then the full tank is 46kg, and 2.8%.

That will have near-as-dammit zero effect on the fuel efficiency of the vehicle.

Now let's pause and think about 33mpg from a full tank, vs 40mpg from 2/3 of a tank.
33mpg from 58 litres is 425 mile range.
40mpg from 2/3 of 58 litres is 345 mile range, or 515 miles from a full tank.
So you'd be getting 80 miles range from 1/3 of 58 litres = 18mpg...

Do we think that realistic? Or do we think that it's far more likely that you mis-measured somehow, probably down to assumptions around the fuel gauge being linear when it isn't...?

There is obviously less thermal mass in a third of a tank compared to a full one, so the same heat input will have a larger effect on the temperature of the content of the tank - but given the tank is underneath the car, out of the sun, protected from the exhaust pipe...

Remember, also, that closed loop petrol injection takes account of the combustion byproducts to adjust the fuelling to stochiometry for the health and efficiency of the catalytic converter.

Diesels regulate the revs and power by adjusting fuel rather than air, but still take account of combustion byproducts to monitor correct fuelling.

If the temperature of the fuel coming in had so much less calorific value per volume injected to be requiring twice as much... you'd definitely be noticing that in how it drove.

The way in which you drive, and the way in which you maintain your car, will have a MUCH greater effect on fuel consumption than worrying about the position of the fuel gauge's needle...

born_again

Only way to get a accurate MPG is the brim to brim method, the more tanks the more accurate it is.

Any single trip info you get is really meaning less, as far to many variables.

I've had 20 mpg up to 175 mpg over the same route & distance in a HEV.

Mildly_Miffed

CliveOfIndia said:

And it's one reason why newer cars generally don't have a spare wheel as standard.

It's how the marketing department spin it... But in reality it's all about cost - with a side order of packaging.

You can't fit the kind of mahoosive alloy that you find on even prosaic superminis (first random example - 95bhp 1.0 Ibiza available with 215/45 18" on 7" rim) under the boot floor of them - so in the very unlikely event you swap a flat for the spacesaver, where do you put that wet and grubby flat...?

The 225/55 18 on my big SUV doesn't even go under the boot floor properly...

There are FAR easier ways to trim weight from that 1,100kg Ibiza than omitting a 3kg spacesaver.

Herzlos

Less weight means more to move, but even if you lose 50l from your tank that's still only about 3% of the cars total empty weight.

It's more complex too because loads of other factors are involve such as aerodynamics, gearing, road surface, gradiants, etc.

So I'm sure the change in economy is completely unrelated to how much fuel is in the tank, with the caveat that if you run the tank low enough you risk dredging up any sediment and making the engine unhappy.

Any savings would be absolutely wiped out by having to visit the petrol station more often to only put in £5.

s71hj

Mildly_Miffed 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.

Let's just pause and think about this...

Your 2010 Zafira is 1430 to 1599kg kerb weight, depending on spec/engine/etc.

It has a 58L tank.
Diesel's density is 850g/litre, petrol is about 800g/litre.

So, assuming you have a 1430kg diesel engined version, a full tank weighs 49kg. That's just 3.4% of the weight of the car...
If it was the opposite end of the spectrum, 1599kg petrol engined, then the full tank is 46kg, and 2.8%.

That will have near-as-dammit zero effect on the fuel efficiency of the vehicle.

Now let's pause and think about 33mpg from a full tank, vs 40mpg from 2/3 of a tank.
33mpg from 58 litres is 425 mile range.
40mpg from 2/3 of 58 litres is 345 mile range, or 515 miles from a full tank.
So you'd be getting 80 miles range from 1/3 of 58 litres = 18mpg...

Do we think that realistic? Or do we think that it's far more likely that you mis-measured somehow, probably down to assumptions around the fuel gauge being linear when it isn't...?

There is obviously less thermal mass in a third of a tank compared to a full one, so the same heat input will have a larger effect on the temperature of the content of the tank - but given the tank is underneath the car, out of the sun, protected from the exhaust pipe...

Remember, also, that closed loop petrol injection takes account of the combustion byproducts to adjust the fuelling to stochiometry for the health and efficiency of the catalytic converter.

Diesels regulate the revs and power by adjusting fuel rather than air, but still take account of combustion byproducts to monitor correct fuelling.

If the temperature of the fuel coming in had so much less calorific value per volume injected to be requiring twice as much... you'd definitely be noticing that in how it drove.

The way in which you drive, and the way in which you maintain your car, will have a MUCH greater effect on fuel consumption than worrying about the position of the fuel gauge's needle...

I'm using full to the brim to the full to the brim technique, not approximately doing it by looking at the fuel gauge.  And I'm on unleaded. 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.