How do cars know how much fuel is in the tank?

I find out the size of the fuel tank from the owners manual. I then fill up a couple of times as soon as the light comes on to know how much fuel I have left once I hit the light
(fuel tank size - fill up amount) then I use my tank average mpg and play make sure I fill up when I have 1-2 litres left in the tank, rather then 7-9litres remaining, which is quite often the case for most modern cars.
 
Syla5 said:
I find out the size of the fuel tank from the owners manual. I then fill up a couple of times as soon as the light comes on to know how much fuel I have left once I hit the light
(fuel tank size - fill up amount) then I use my tank average mpg and play make sure I fill up when I have 1-2 litres left in the tank, rather then 7-9litres remaining, which is quite often the case for most modern cars.
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Seems a bit foolish, do we have permission to laugh at you when you inevitably run out one day?
 
Syla5 said:
I find out the size of the fuel tank from the owners manual. I then fill up a couple of times as soon as the light comes on to know how much fuel I have left once I hit the light
(fuel tank size - fill up amount) then I use my tank average mpg and play make sure I fill up when I have 1-2 litres left in the tank, rather then 7-9litres remaining, which is quite often the case for most modern cars.
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Is it not recommended that you don't let your fuel get too low (like nearly empty) as it will pick up all the crap at the bottom of the tank to go through your engine?

or is that an old wives tale. :p
 
Clarkey said:
Seems a bit foolish, do we have permission to laugh at you when you inevitably run out one day?
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To be fair as "proved" on Top Gear several times before, when the trip computer and even the fuel gauge say 0 miles or empty, there can still be good few miles left in the tank.

With regards to the whole grot at the bottom of the tank thing - cant see how it can be an issue these days... the fuel pickup has to be virtually at the bottom of the tank, to make use of the full tank size... fuel tank materials and production processes have moved on since the days of rust being common in fuel tanks (thinking classic minis etc here), and have had to be due to the move to EFI and Direct Injection.

As I understand it, the size of the injector nozzles are tiny, and so any Grot getting through would cause major problems - hence the filters in/on the fuel pump, and further fuel filters before the injector system.

Fuel pumps overheating when low on fuel... again I think if this was a problem then there would be a big disclaimer in every owners manual to state such... otherwise think of the lawsuits...
 
Clarkey said:
Seems a bit foolish, do we have permission to laugh at you when you inevitably run out one day?
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I've always done this - standing in a petrol station bores me to death so generally I fill when the remaining range is 15 miles or so, which is under 4 litres left.
 
[TW]Fox;23742695 said:
The ECU knows how many litres are left and it also knows, on average, how much fuel its using. It can then calculate the expected range from this. You can often see a lot of this raw data yourself if you have the right interface/software :)
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That's not strictly true is it. I don't think the ECU can know exactly how many litres are in the tank as the float can only hit max within the tank (not the filler neck for example). It's more a calculation of miles driven vs rough amount of fuel left given the float position. Part of the reason it's always woefully inaccurate, especially for cars that live permanently in urban conditions
 
Jono8 said:
Is it not recommended that you don't let your fuel get too low (like nearly empty) as it will pick up all the crap at the bottom of the tank to go through your engine?

or is that an old wives tale. :p
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Did you even read the rest of the thread?
 
[TW]Fox;23745373 said:
I've always done this - standing in a petrol station bores me to death so generally I fill when the remaining range is 15 miles or so, which is under 4 litres left.
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Only 4l? Yesterday I happened to run mine really low, about 20 miles after the range went to zero and the gauge off the bottom, still only managed to put 56l in a 62l tank. Normally I'd just fill it when the light comes on which is about 1/8th tank left. I had planned on running it down low and filling with SUL having read the loltastic s2000 thread, just to see if it made any difference. I saw the price though, and then filled with regular.
 
It seems reasonably accurate - had range of 15 miles the other week which is about 3 litres worth, and it took 66.something litres.
 
Clarkey said:
Seems a bit foolish, do we have permission to laugh at you when you inevitably run out one day?
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Explain why its foolish? You clearly havent based that assumption on my day to day journey or driving habbits? Considering i pass 6 petrol stations on my 7 mile commute, at regular intervals as well!! and i get 8 miles from 1 litre of fuel, topping up with between 8-16 miles left in the tank is quite a margin for error when i only have to go 1-2 miles to fill up at most. Also i havent run out of fuel yet in my 12 years of driving, so i guess my method is clearly flawed and that i am indeed foolish :p

Jono8 said:
Is it not recommended that you don't let your fuel get too low (like nearly empty) as it will pick up all the crap at the bottom of the tank to go through your engine?

or is that an old wives tale. :p
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Never had a problem, i have run all my cars this way from my 1990 renault 5 gtturbo to my 58 plate focus st. Very different fuel systems, no issues.

Also something about this really does bother my. Where does the fuel pump draw the fuel from in the tank, surely any feed is going to be from the bottom of the tank anyway, so you would get this issue regardless of fuel level, if you had serious gunk in the tank??? I just let the fuel filters do there job.
 
Lucero said:
That's not strictly true is it. I don't think the ECU can know exactly how many litres are in the tank as the float can only hit max within the tank (not the filler neck for example). It's more a calculation of miles driven vs rough amount of fuel left given the float position. Part of the reason it's always woefully inaccurate, especially for cars that live permanently in urban conditions
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What if the tank doesn't just use a float gauge?
 
Howard said:
Me too and I don't even know why. It doesn't even take that long. But I wish my car had a bigger tank.
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It's not the standing that bothers me. It's watching the blinkin numbers spin round to a bigger number every time I fill up. It seems daily there's a change in price. 1979 when I started driving it was 0.98p a gallon.
 
It's only explosive given the right air/fuel ratio, which is quite hard to achieve in a fuel tank, in addition the sender and pump are designed to be run in flammable environments.
 
The computer or a float sensor...

In our, C5, our petrol levels go ''down'' even when driving on LPG, the computer likes to calculate the range... Then once it's completely dry, after a while it'll reset itself and it's back ''full'' again. Stupid system imo but meh. It also resets itself when you've refuelled, think there's some sensor that knows when you refuel, but even when not touching petrol you can see the range going down from 300km to 0 then back to 300 after a while when only driving on LPG :).

Most cars simply have a floater though, in some cars like a 106 it's actually hilarious, one moment you're bombing through a corner and the petrol light goes on as it's nearly ''empty'', the next moment you have half a tank when driving calmly :D.
 
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