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

born_again

s71hj said:

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.

So how do you know you are getting better MPG from a half full tank, than a full one? If you are Brim to Brim filling?

s71hj

born_again said:

s71hj said:

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.

So how do you know you are getting better MPG from a half full tank, than a full one? If you are Brim to Brim filling?

So if I fill up (know i have because  the pump stops working) set my trip to 0 miles, when I fill up again until the pump stops working I know how many miles I've done and how much fuel is has taken to get it back to full.

Goudy

Okay, I see what you are doing here.

You are filling the car, running it until it's 2/3th full.
Refilling it and working out the MPG based on miles/litres to refill.

Then filling the car running it until it's near empty.
Refilling it and working out the MPG based on miles/litres to refill.

And now you are wondering why you get two different figures for each test.

Well the second "test" is over a greater distance.

I could do the same thing and get a difference of 30 mpg or so.
If I did the first test down the motorway for 100 miles I would get 58-60 mpg.
Then if I did the second test around town, I'd get closer to 30 mpg.

paul_c123

s71hj said:

born_again said:

s71hj said:

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.

So how do you know you are getting better MPG from a half full tank, than a full one? If you are Brim to Brim filling?

So if I fill up (know i have because  the pump stops working) set my trip to 0 miles, when I fill up again until the pump stops working I know how many miles I've done and how much fuel is has taken to get it back to full.

Yeah that's how you do full-full, but the question related to how to compare mpg from a half-full tank vs a full tank.

Also, there's far too much variance in when the filler clicks to prevent overfilling. Really you need to manually keep going again and again after the first click, to actually see the fuel brimmed to the bottom of the fuel cap opening.

And even then, it will depend on the temperature of the fuel as to how much (by weight) is actually in there, becaus density changes by temperature (quite a lot). Its not even as simple as the ambient temperature, because the temperature the fuel is stored at in the bunker may be less than that. And if the tank isn't empty and the car has been running a while, its quite possible that some of that fuel in it is a higher temperature, because its been circulating around the engine area due to the fuel return line (some cars have a fuel return, some don't).

Given all the above, and unless the fuel tank is very large capacity, I'd say the extra weight of the fuel is insignificant in economy. Much more significant is type of roads driven; efficient driving (including windows open/closed, aircon use); mechanical health of car; tyre pressures; outside temperature; extra weight eg payload, passengers carried.

s71hj

Goudy said:

Okay, I see what you are doing here.

You are filling the car, running it until it's 2/3th full.
Refilling it and working out the MPG based on miles/litres to refill.

Then filling the car running it until it's near empty.
Refilling it and working out the MPG based on miles/litres to refill.

And now you are wondering why you get two different figures for each test.

Well the second "test" is over a greater distance.

I could do the same thing and get a difference of 30 mpg or so.
If I did the first test down the motorway for 100 miles I would get 58-60 mpg.
Then if I did the second test around town, I'd get closer to 30 mpg.

Yes but I'm getting a big difference in mpg (well appx 10 to 15 mpg, maybe it's a matter of debate if that is "big" I'm beginning to think!)from journeys that have pretty much the same proportion of motorway/ dual carriageway and urban driving. Aircon never on, windows never open, no significant weight change in the  vehicle.

paul_c123

s71hj said:

Goudy said:

Okay, I see what you are doing here.

You are filling the car, running it until it's 2/3th full.
Refilling it and working out the MPG based on miles/litres to refill.

Then filling the car running it until it's near empty.
Refilling it and working out the MPG based on miles/litres to refill.

And now you are wondering why you get two different figures for each test.

Well the second "test" is over a greater distance.

I could do the same thing and get a difference of 30 mpg or so.
If I did the first test down the motorway for 100 miles I would get 58-60 mpg.
Then if I did the second test around town, I'd get closer to 30 mpg.

Yes but I'm getting a big difference in mpg (well appx 10 to 15 mpg, maybe it's a matter of debate if that is "big" I'm beginning to think!)from journeys that have pretty much the same proportion of motorway/ dual carriageway and urban driving. Aircon never on, windows never open, no significant weight change in the  vehicle.

Can you show your raw data?

born_again

paul_c123 said:

s71hj said:

born_again said:

s71hj said:

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.

So how do you know you are getting better MPG from a half full tank, than a full one? If you are Brim to Brim filling?

So if I fill up (know i have because  the pump stops working) set my trip to 0 miles, when I fill up again until the pump stops working I know how many miles I've done and how much fuel is has taken to get it back to full.

Yeah that's how you do full-full, but the question related to how to compare mpg from a half-full tank vs a full tank.

Also, there's far too much variance in when the filler clicks to prevent overfilling. Really you need to manually keep going again and again after the first click, to actually see the fuel brimmed to the bottom of the fuel cap opening.

And even then, it will depend on the temperature of the fuel as to how much (by weight) is actually in there, becaus density changes by temperature (quite a lot). Its not even as simple as the ambient temperature, because the temperature the fuel is stored at in the bunker may be less than that. And if the tank isn't empty and the car has been running a while, its quite possible that some of that fuel in it is a higher temperature, because its been circulating around the engine area due to the fuel return line (some cars have a fuel return, some don't).

Given all the above, and unless the fuel tank is very large capacity, I'd say the extra weight of the fuel is insignificant in economy. Much more significant is type of roads driven; efficient driving (including windows open/closed, aircon use); mechanical health of car; tyre pressures; outside temperature; extra weight eg payload, passengers carried.

Which is the point I'm making.
How can the OP know the difference, if they are only doing brim to brim? Which they keep stating.🤦‍♀️


All I can think is they are looking at trip MPG which really is a pointless figure. Due to, too many variables in each trip.

s71hj

born_again said:

paul_c123 said:

s71hj said:

born_again said:

s71hj said:

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.

So how do you know you are getting better MPG from a half full tank, than a full one? If you are Brim to Brim filling?

So if I fill up (know i have because  the pump stops working) set my trip to 0 miles, when I fill up again until the pump stops working I know how many miles I've done and how much fuel is has taken to get it back to full.

Yeah that's how you do full-full, but the question related to how to compare mpg from a half-full tank vs a full tank.

Also, there's far too much variance in when the filler clicks to prevent overfilling. Really you need to manually keep going again and again after the first click, to actually see the fuel brimmed to the bottom of the fuel cap opening.

And even then, it will depend on the temperature of the fuel as to how much (by weight) is actually in there, becaus density changes by temperature (quite a lot). Its not even as simple as the ambient temperature, because the temperature the fuel is stored at in the bunker may be less than that. And if the tank isn't empty and the car has been running a while, its quite possible that some of that fuel in it is a higher temperature, because its been circulating around the engine area due to the fuel return line (some cars have a fuel return, some don't).

Given all the above, and unless the fuel tank is very large capacity, I'd say the extra weight of the fuel is insignificant in economy. Much more significant is type of roads driven; efficient driving (including windows open/closed, aircon use); mechanical health of car; tyre pressures; outside temperature; extra weight eg payload, passengers carried.

Which is the point I'm making.
How can the OP know the difference, if they are only doing brim to brim? Which they keep stating.🤦‍♀️


All I can think is they are looking at trip MPG which really is a pointless figure. Due to, too many variables in each trip.

I am looking at mpg across full to 1/2 full then I'm looking at mpg across full to much closer to empty. I didn't think I'd in any way suggested otherwise. Repeatedly full to around half full appears to show a better mpg when (in so far as it is possible) other factors are roughly the same eg no aircon,  windows shut, car not carrying notable extra weight, similar journey type. As I originally said, I'd expected if anything the opposite. As I said I read something online that noted this as a phenomenon in older cars,  but as I also said you can read a lot on the internet! Given there is a potential moneysaving element in terms of expenditure on fuel I thought here may be a reasonable place to ask the question. 

Car_54

s71hj said:

Goudy said:

Okay, I see what you are doing here.

You are filling the car, running it until it's 2/3th full.
Refilling it and working out the MPG based on miles/litres to refill.

Then filling the car running it until it's near empty.
Refilling it and working out the MPG based on miles/litres to refill.

And now you are wondering why you get two different figures for each test.

Well the second "test" is over a greater distance.

I could do the same thing and get a difference of 30 mpg or so.
If I did the first test down the motorway for 100 miles I would get 58-60 mpg.
Then if I did the second test around town, I'd get closer to 30 mpg.

Yes but I'm getting a big difference in mpg (well appx 10 to 15 mpg, maybe it's a matter of debate if that is "big" I'm beginning to think!)from journeys that have pretty much the same proportion of motorway/ dual carriageway and urban driving. Aircon never on, windows never open, no significant weight change in the  vehicle.

Any savings you make on fuel will come in handy when you have to repair the air-con. The consequences of prolonged non-use are well-known.

s71hj

Car_54 said:

s71hj said:

Goudy said:

Okay, I see what you are doing here.

You are filling the car, running it until it's 2/3th full.
Refilling it and working out the MPG based on miles/litres to refill.

Then filling the car running it until it's near empty.
Refilling it and working out the MPG based on miles/litres to refill.

And now you are wondering why you get two different figures for each test.

Well the second "test" is over a greater distance.

I could do the same thing and get a difference of 30 mpg or so.
If I did the first test down the motorway for 100 miles I would get 58-60 mpg.
Then if I did the second test around town, I'd get closer to 30 mpg.

Yes but I'm getting a big difference in mpg (well appx 10 to 15 mpg, maybe it's a matter of debate if that is "big" I'm beginning to think!)from journeys that have pretty much the same proportion of motorway/ dual carriageway and urban driving. Aircon never on, windows never open, no significant weight change in the  vehicle.

Any savings you make on fuel will come in handy when you have to repair the air-con. The consequences of prolonged non-use are well-known.

It's never worked since we got the car