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

CliveOfIndia

s71hj said:

I am looking at mpg across full to 1/2 full then I'm looking at mpg across full to much closer to empty.

My money is on the fuel gauge.  They are notoriously inaccurate (I suppose they can't really be anything but, due to how they work).  They are designed to give a rough indication of how much is left, but if you're talking about looking at when the fuel gauge shows "half full", that could easily vary by a couple of gallons from one day to the next.

On my wife's car, on more than one occasion the gauge shows 2 bars and is red, meaning it's getting very low.  Turn off the engine, get back in half an hour later and it's up to 3 or 4 bars.  Temperature, fuel sloshing around in the tank, simply a margin of error on the sender unit, being parked on a slight slope, I don't know.  But if your gauge is showing "half full", I'd bet big money that it's not exactly half full, even if the gauge appears to show such.

s71hj

CliveOfIndia said:

s71hj said:

I am looking at mpg across full to 1/2 full then I'm looking at mpg across full to much closer to empty.

My money is on the fuel gauge.  They are notoriously inaccurate (I suppose they can't really be anything but, due to how they work).  They are designed to give a rough indication of how much is left, but if you're talking about looking at when the fuel gauge shows "half full", that could easily vary by a couple of gallons from one day to the next.

On my wife's car, on more than one occasion the gauge shows 2 bars and is red, meaning it's getting very low.  Turn off the engine, get back in half an hour later and it's up to 3 or 4 bars.  Temperature, fuel sloshing around in the tank, simply a margin of error on the sender unit, being parked on a slight slope, I don't know.  But if your gauge is showing "half full", I'd bet big money that it's not exactly half full, even if the gauge appears to show such.

I'm not going by where the fuel indicator is. I'm going by how much I actually put in to get it back to full and  how many miles my trip gauge that I set back to zero says I've done.

sheslookinhot

I fill up and enjoy driving the car, mpg will be what it is, nothing I can do about it. I don’t waste time calculating mpg, what’s the point ?

s71hj

sheslookinhot said:

I fill up and enjoy driving the car, mpg will be what it is, nothing I can do about it. I don’t waste time calculating mpg, what’s the point ?

That's perfectly reasonable. I was asking the question as there is a Moneysaving element potentially 

paul_c123

TBH, while the trip computer fuel economy reading might not give terribly accurate results, I believe it will be useful to show relative results. You can modify your driving and see the results on the trip computer.

I once persuaded my Golf to do 67mpg on a trip to work (25 miles, mainly A-roads and a stretch of motorway). Its more usual result was ~60mpg.

Google "hypermiling", but don't blindly follow all the techniques, some of them compromise safety for fuel economy. But certainly you can use some of them to improve fuel economy.

maxmycardagain

full to full/miles covered over 10 tank fulls is the only true way to obtain an accurate MPG figure

https://www.spritmonitor.de/en/

I drove Calais to Roses (Spain) on 1 tank of diesel (Skoda Superb 1.6 estate)

born_again

s71hj said:

born_again said:

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. 

As I said. 
How do you know that the gauge at half full has used half the tank oof fuel?

A example is Kia's
They will take a age to get to half a tank. Then drop like a brick to empty. Is their MPG worse in the bottom half?
No.
Old HEV used to show (GOM) 500 to empty. At half would show 300, then drop like a brick to empty.

It's not a phenomenon, it physics. But you would save even more with a lighter foot, or driving down hill with a following wind.

s71hj

born_again said:

s71hj said:

born_again said:

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. 

As I said. 
How do you know that the gauge at half full has used half the tank oof fuel?

A example is Kia's
They will take a age to get to half a tank. Then drop like a brick to empty. Is their MPG worse in the bottom half?
No.
Old HEV used to show (GOM) 500 to empty. At half would show 300, then drop like a brick to empty.

It's not a phenomenon, it physics. But you would save even more with a lighter foot, or driving down hill with a following wind.

Whether the gauge looks appx half full, near empty etc plays no part in the calculations. The number of litres to get it back to full and the number of miles driven since it was previously full are the elements that are used for mpg calculation.

Ditzy_Mitzy

Mildly_Miffed said:

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.

I'm inclined to agree with @Sarahspangles as the silt issue has affected me in the past.  I let the petrol run rather low in a car a long time ago and that did cause running issues for a while afterwards; seems that gunk got sucked out of the petrol tank and lodged in the carburettor.  The car in question was a Ford Cortina, so not sure how relevant this is to today's context, however!  

Can't say I've noticed any difference in fuel economy with a full or empty tank, though!  Perhaps it was truer in the past; do seem to remember that cars would misfire when petrol was low, so perhaps more throttle was used?

prowla

I can drive my car for miles along the motorway and it shows more range than when I started.