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the reason not to fill a modern car to the brim is that they have a charcoal canister designed to stop fumes from the fuel escaping into the atmosphere

if you fill a tank to the brim you flood this filter with fuel and damage it, it is part of the emission control system
 

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the reason not to fill a modern car to the brim is that they have a charcoal canister designed to stop fumes from the fuel escaping into the atmosphere

if you fill a tank to the brim you flood this filter with fuel and damage it, it is part of the emission control system
Also the fuel tank is pressurised, and filling to brim might upset this.
 

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I rather fill the tank up to full capacity at a petrol station low prices as in my area usually once or twice a month they have low prices until they rise again. You could save yourself between £5-£10 per month by filling to capacity at a supermarket fuel offer.
And spend about £5 a month hauling all that unnecessary fuel around.

A litre of petrol weighs 0.737 kg or 1.62 lbs.

45 litres will therefore weigh about 33 kilos or about 72 lbs. It's a descending margin as fuel is slowly used but it is NOT a good idea to carry fuel around because it's cheap, only because it's needed. A supermarket offer will usually cover all their stores from Abergavenny to Aberdeen and will be subject to regional variation so it may actually be cheaper to buy fuel a long way from home as well as cheaper to only carry what you need...

Have a look at the forum posts about weight. It DOES affect fuel consumption. :nerd:
 
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Average weight of tank is only slightly increased by adding a liter or two. One of the cool things about our cars is the range reducing the number of times we have to fill the tank. Nobody likes this chore.

Call me a skeptic about the ease of damaging mandated emissions systems. If that was true, then governments would mandate improvements. Check valves are cheap, as is a simple fuel line loop higher than the top of the tank (physics) to prevent any contamination of a charcoal filter. If any idiot (me if you like) can destroy an emissions systems by simply topping off, something is very wrong indeed.

Trusting first click? Which pump? On my last car, first click varied between 1 and a half US gallons to over 3 US gallons before full. In the US back pressure turn off calibration of fuel pumps is all over the place and it is hard to imagine consistency between pumps anywhere.
 

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Average weight of tank is only slightly increased by adding a liter or two. One of the cool things about our cars is the range reducing the number of times we have to fill the tank. Nobody likes this chore.

Call me a skeptic about the ease of damaging mandated emissions systems. If that was true, then governments would mandate improvements. Check valves are cheap, as is a simple fuel line loop higher than the top of the tank (physics) to prevent any contamination of a charcoal filter. If any idiot (me if you like) can destroy an emissions systems by simply topping off, something is very wrong indeed.

Trusting first click? Which pump? On my last car, first click varied between 1 and a half US gallons to over 3 US gallons before full. In the US back pressure turn off calibration of fuel pumps is all over the place and it is hard to imagine consistency between pumps anywhere.
Topping off can potentially force fuel into the tubes that lead to the charcoal filter. It's not certainly going to happen but it's a risk nonetheless so I always stop at the first click. You're absolutely right that the back pressure turn off calibration for fuel pumps is all over the board though. That's why people who track their tank to tank fuel economy very carefully always try to use the same dispenser every time. That's not practical for most people though. What would stop people from continuing to top off if a different system besides the back pressure system was implemented? There is a system in place now designed to prevent people from damaging these systems but it's up to the users to not override it, just like it's up to the user to fasten their seat belt or keep the accident avoidance systems turned on.
 

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Average weight of tank is only slightly increased by adding a liter or two. One of the cool things about our cars is the range reducing the number of times we have to fill the tank. Nobody likes this chore.

Call me a skeptic about the ease of damaging mandated emissions systems. If that was true, then governments would mandate improvements. Check valves are cheap, as is a simple fuel line loop higher than the top of the tank (physics) to prevent any contamination of a charcoal filter. If any idiot (me if you like) can destroy an emissions systems by simply topping off, something is very wrong indeed.

Trusting first click? Which pump? On my last car, first click varied between 1 and a half US gallons to over 3 US gallons before full. In the US back pressure turn off calibration of fuel pumps is all over the place and it is hard to imagine consistency between pumps anywhere.
Chore? I do love it when I save more money and only stop as often. It's not the difference of a litre or two, its the difference between 10 and 45 litres.

I plan my distant journeys with military precision. Even if you have fuel in the tank it's no advantage when you have to stop for coffee, a pee, or shopping - because I hate to waste a journey with unnecessary stops, but can appreciate and plan for the necessary ones.

I'm on a three stop strategy tomorrow and planned this mornings refuelling to make the third stop a refuel stop for me, the car, and our fridge . Anything less is simply inefficient and I have a home and alcohol to get back to...

Filling up the car is simply a lazy choice, without contemplation of the bigger picture. I regularly get my wine from my local petrol station. I get a bottle every Friday. Why on earth would I carry fuel to see me through to Saturday?
 

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Filling up the car is simply a lazy choice, without contemplation of the bigger picture. I regularly get my wine from my local petrol station. I get a bottle every Friday. Why on earth would I carry fuel to see me through to Saturday?
Not a lazy choice for me . Maybe it's a throwback to my young days when I could barely afford fuel always ran on fumes and often the fumes ran out :eek: . Too old to walk miles with a petrol can in my hand any more . The other thing is it caters for the unexpected detours , hold ups ,journeys and turning up at a petrol station that's run out of fuel that you can't plan for . I only pass one garage on my commute and that's renowned for for selling duff fuel .
As I posted earlier in the thread I've found little , if any , impact on my fuel efficiency whether running on a fuel or half tank . I understand the logic that if I'm carrying less weight so I should use less fuel , but it doesn't seem to apply in my case . I once did a 45 mile motorway trip carrying one passenger and about 500 pounds of books in the back and still returned over 80 mpg
 

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Chore? I do love it when I save more money and only stop as often. It's not the difference of a litre or two, its the difference between 10 and 45 litres.

I plan my distant journeys with military precision. Even if you have fuel in the tank it's no advantage when you have to stop for coffee, a pee, or shopping - because I hate to waste a journey with unnecessary stops, but can appreciate and plan for the necessary ones.

I'm on a three stop strategy tomorrow and planned this mornings refuelling to make the third stop a refuel stop for me, the car, and our fridge . Anything less is simply inefficient and I have a home and alcohol to get back to...

Filling up the car is simply a lazy choice, without contemplation of the bigger picture. I regularly get my wine from my local petrol station. I get a bottle every Friday. Why on earth would I carry fuel to see me through to Saturday?
This from the person with the company car, who says he doesn't worry about trying to drive economically as it's not his petrol money. >:)

I always suspected there was just a hint of environmental frugalist in you SEvans! >:)

Seriously though, I have my fuel economy gauge set to reset after each fill-up (my personal choice), and it always seems better to start with gradually worsens as the tank gets closer to empty? Maybe it's just because I'm trying to drive more carefully after just filling up...I don't know. But, and I know this seems counter-intuitive, (but I even remember this in my last car), I seem to get better economy when I have passengers in the car, than when it's just me. The only thing I can possibly attribute it to is weight distribution; sure there's more weight in the car, but more evenly distributed. I'll admit, I'm a somewhat heavy individual to begin with (although getting lighter—a lot from where I once was), so I don't know if that contributes to it at all.
 
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I don't know the official reason about the air gap, but I was in the Army and we had hundreds of vehicles in hangars, they were all filled to the brim, this I was told is because the vapour mixture in a tank is explosive, where as a full tank with no vapour is less likely to explode! BUT!!!! on a hot day we had to open all the hangar doors to vent fumes from all the fuel dripping on to the floor, as it expanded in the tanks!
 

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I don't know the official reason about the air gap, but I was in the Army and we had hundreds of vehicles in hangars, they were all filled to the brim, this I was told is because the vapour mixture in a tank is explosive, where as a full tank with no vapour is less likely to explode! BUT!!!! on a hot day we had to open all the hangar doors to vent fumes from all the fuel dripping on to the floor, as it expanded in the tanks!
Hot Day ? Wales ?....you must have been posted elsewhere ;)
 

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Veering off Ioniq topic a bit, but commenting on 'venting', on VW diesels like my '02 Jetta TDI, there's a venting tab just inside the filler neck, that when pressed and held with a Popsicle stick or something similar while continuing to fill, I can get nearly two more gallons after the filler auto cut-off. This translates to 70-80 more miles in the Jetta TDI and have never had an issue 'filling to the brim'. I never fill it until fuel is running out the overfill, but pretty close.

The TDI has been a great, reliable car, but the rising cost of diesel is the reason I'm considering an Ioniq.
 

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Just because a particular VW model "apparently" has not had a problem doesn't mean it translates across to all makes and models of cars.

After all VW "doctored" their emissions, but this does not mean ALL manufacturers did.
 

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Most manufacturers do game emissions and efficiency. Since the 1970s.
 

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Thanks for all the discussion about the air gap. I usually drive a bit after filling so hopefully that also uses up some of the gas quickly. I agree that different pumps switch off more easily than others, but I would like to get as consistent a refill each time as possible. That let's me calculate the real mileage, instead of what the car is telling me.

I have two trip meters in my Ioniq Hybrid (USA), so I always clear "Trip 2" when I refill. I'm a new owner, and have only filled the car twice so far in a little over 2 months of driving. As it gets warmer here, I expect my mileage will improve a little more too.
 

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Thanks for all the discussion about the air gap. I usually drive a bit after filling so hopefully that also uses up some of the gas quickly. I agree that different pumps switch off more easily than others, but I would like to get as consistent a refill each time as possible. That let's me calculate the real mileage, instead of what the car is telling me.

I have two trip meters in my Ioniq Hybrid (USA), so I always clear "Trip 2" when I refill. I'm a new owner, and have only filled the car twice so far in a little over 2 months of driving. As it gets warmer here, I expect my mileage will improve a little more too.

The two `accurate`measurements for fuel consumption are the reading on the pump and the distance showing on the car - not necessarily scientifically accurate, but it will be consistent. A website like Fuelly allows one to enter those figures to gauge an assessment of mileage to a greater degree of accuracy. :nerd:
 
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This discussion is interesting. It seems the British are more concerned about their fuel systems than folks in the US. That may be due to high cost of petrol there! I have rarely driven so long that the low fuel light comes on. I usually fill up when I have less than half a tank or when I passing my favorite fuel stop. I have never noticed any degradation in economy with a full tank. I also have never noticed fewer MPG when my wife rides with me either! Fuel here is about $1.85/gal right now. At 11.9 gal tank size that would mean it would cost me about $22 to fill up. But I have never put that much fuel in. My average fill up is about 8 gal. and that is about once every two weeks or so.
 

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I usually fill up when I have less than half a tank or when I passing my favorite fuel stop. I have never noticed any degradation in economy with a full tank.
You would never be able to notice an mpg drop of an extra 100 or even 200 pounds of intermittent added weight with low fill volumes (half tank) fill. But if every car on the road added that amount of weight full time there would be a notable extra amount of fuel used worldwide, despite very small differences to an individual owner.

The point being, just because you cannot notice a difference, doesn't mean there isn't one. Similar logic to reported US cases of Covid-19 would drop if we stopped testing (per Trump). Technically correct as is your not noticing an mpg drop, but just as wrong. Part of being green is making choices about buying hybrids instead of SUVs, perhaps not carrying a spare or other added weight all the time, and perhaps not filling as often to reduce average weight. Personal choices to be sure and I'm not the green police, but don't dismiss out of hand the impact of extra weight. You can ignore physics, but physics doesn't care about your opinion.
 

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I have often wondered why a KIA driver keeps posting on an Ioniq forum! Don't Kia's have forums???
 

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This discussion is interesting. It seems the British are more concerned about their fuel systems than folks in the US. That may be due to high cost of petrol there! I have rarely driven so long that the low fuel light comes on. I usually fill up when I have less than half a tank or when I passing my favorite fuel stop. I have never noticed any degradation in economy with a full tank. I also have never noticed fewer MPG when my wife rides with me either! Fuel here is about $1.85/gal right now. At 11.9 gal tank size that would mean it would cost me about $22 to fill up. But I have never put that much fuel in. My average fill up is about 8 gal. and that is about once every two weeks or so.
Like you I've never noticed any difference in fuel economy when the car is fully laden . Makes no sense really but it seems to be the case after driving an Ioniq for 3 and half years . I do have a theory but I'm keeping it to myself ;)
 
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