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My car has a 52.3 litres fuel tank but why does it only take 18 litres from half way

I have a 2006 mk2 focus sport 1.8 TDCI 5 door

In the paperwork reads fuel tank capacity 52.3 litres, but recently I notice that when the fuel needle in my car is around half way line, I top up and fill it to the brim but it only takes 18 litres, surely the 1st half of fuel before halfway line can't contain around 34 litres ? also the lower half of fuel seems to burn faster than the fuel over the half way line so I can't see the bottom half having 34 litres

any ideas ?

Reply 1

dhr90

Te fuel needle gauge is rarely accurate. When it says it is empty there is usually a couple of litres sill available as absolute emergency.

Posted from TSR Mobile

Reply 2

uberteknik

Original post by I love life

It's a function of the device used to measure the tank contents which is usually a float connected to the wiper of a variable resistance which has a non-linear (logarithmic-scale) track.

Since these components are not very accurate (typically plus or minus 10 to 20%), they will only ever provide an estimate of the fuel in the tank.

Reply 3

I love life

Original post by uberteknik

Original post by dhr90

Te fuel needle gauge is rarely accurate. When it says it is empty there is usually a couple of litres sill available as absolute emergency.

Posted from TSR Mobile

So just before the warning light comes on to full brim takes about 40 litres, but my tank capacity is 52.3 litres, are you saying that I have almost 13 litres of fuel when warning light comes on ?

I don't think so

Maybe a smaller fuel tank has been installed ?

Reply 4

Hippysnake

Like carbohydrates in your stomach, fuel expands when it enters your tank as it mixes with the small amount of air present in tank. It's known as air dilution of a solution (see Son-Goku's theory on solution dilution).

Source: I have a theoretical degree in Physics.

Reply 5

I love life

Original post by Hippysnake

I'm asking, shouldn't it take 26 litres ( that is half the 52.3 litres my car reportedly hold of fuel) to fill my car from half way point, not 18 litres ?

Reply 6

Hippysnake

Original post by I love life

I'm asking, shouldn't it take 26 litres ( that is half the 52.3 litres my car reportedly hold of fuel) to fill my car from half way point, not 18 litres ?

You're not taking into account air dilution. As the diesel or petrol mixes with air, 18 litres can expand. In some cases, the dilution can be 50%, so 27 litres wouldn't be unexpected. this is all complete bull**** btw

Reply 7

I love life

Original post by Hippysnake

not a clue what your banging on about, but meh

anyway, so although the fuel capacity for my car is 52.3 litres, your saying I only have a 40 ltr capacity ?

Reply 8

mackemforever

Original post by I love life

I'm asking, shouldn't it take 26 litres ( that is half the 52.3 litres my car reportedly hold of fuel) to fill my car from half way point, not 18 litres ?

Yes, if your fuel tank was exactly half full then it would take another 26 litres to completely fill it, however when you think the fuel gauge is showing it as half full it's actually not.

What happens when you're fuel gauge shows empty? Does your car immediately grind to a halt, or does it continue to work fine for a while?

Reply 9

I love life

Original post by mackemforever

I filled it from half way to the brim and it only took 18 litres, not 26.

are you saying the fuel sender unit is faulty ?

A fuel gauge tells you how much fuel you have in your tank, it's not going to be £16 wrong i.e. saying I have 16 ltrs less then I actually do

Reply 10

mackemforever

Original post by I love life

Ok, listen.

Fuel gauges have never been accurate and as they still use a mechanism that has been largely unchanged for 50 years they are still not accurate. They give you an approximation, nothing more. If it says your tank is half full then it is approximately half full. If it says your tank is empty then it is approximately empty.

Reply 11

I love life

Okay, so when I fill my car from empty to full and achieve 400 miles, is that 400 miles from 40 ltrs or 52.3 ?
it only takes 40 ltrs from empty to full

If you didn't want to help you shouldn't have bothered trying too, especially if your going to have a fit and get extremely angry , and besides you didn't even help, just confused me but I think I'll disregard your information anyway because I call troll, you don't seem to know what your talking about

Reply 12

Nuffles

Original post by I love life

Christ you're about as thick as a wooden pole.

If you emptied your tank - as in drove until the car died or siphoned it out, then it would take ~50 litres to fill it. However, you never actually empty your tank because diesel systems are a pain to prime properly once they've run dry.

In answer to your first question, you get 400 miles from the litres displayed on the fuel pump when you fill the car up.

Just get a Fuelly account like I've told you several times before and you'll have all the information you need.

Reply 13

uberteknik

Original post by I love life

If you are trying to work out your fuel consumption accurately, then do this:

1) Take your car to a level forecourt and fill up your tank as far as you can. This will be when the fuel is almost all the way up to the filler cap hose and you can see the fuel in it. You will need to let the fuel settle several times as it will foam up and stop the pump. Let the fuel settle and depress the hose slowly when this starts to happen. Keep doing this whilst slowly removing the nozzle until you can see the fuel rising in the filler cap entry pipe. Your tank is now full to the brim.

2) Reset your trip counter to zero before you leave the forecourt.

3) Drive your car as you would normally. When the petrol gauge shows somewhat less than half full, take the car to a petrol station and fill up the tank as you did in step 1).

4) Make a note of exactly how many litres it took to fill the tank the second time like this. i.e. make a note of how many litres (including decimal places) were purchased and registered at the pump when the tank is filled in this way.

5) Read the trip counter before starting the engine and leaving the forecourt.

6) Your fuel consumption is then calculated from the trip counter mileage recorded divided by litres of fuel purchased at the second filling. i.e. step 5 reading divided by step 4 reading.

The answer will be in miles per litre.

As I and other have said, fuel gauges are only an estimate for the contents of the tank and cannot be relied upon to be accurate. However, the car manufacturer will state the total tank capacity and this must be regarded as an accurate figure.

(edited 8 years ago)

Reply 14

CurlyBen

Original post by uberteknik

Original post by mackemforever

Ok, listen the **** up.

Fuel gauges have never been accurate and as they still use a mechanism that has been largely unchanged for 50 years they are still not accurate. They give you an approximation, nothing more. If it says your tank is half full then it is approximately half full. If it says your tank is empty then it is approximately empty.

Why the **** does it matter? If your car has 20 litres of empty space in the fuel tank, then it will cost you £20 to fill. Does that cost change whether the ****ing fuel ****ing gauge ****ing says the ****ing fuel ****ing tank is ****ing half ****ing empty or not? No it ****ing doesn't.

Now ****ing **** off you ****ing annoying ****ing ****.

Just as a point of interest, not all fuel senders are that design any more and I'd probably say most aren't. The last one I looked at used a float with a magnet in it to close reed switches, which has the advantage of being non-contact. It does mean there are only eight outputs though (1/8, 1/4, 3/8 etc.) - it's effectively a digital gauge, even though the readout is analogue.