Fuel gauge showing incorrect level.

hi. i have recently been gifted an 05 plate 1.4 sx from a family member. it had 2 lights showing on the guage, but, whilst the pump prices were so low i thought i'd fill it. went to asda and stopped filling as soon as pump clicked off. when i left asda had 6 lights on...sorted. however with not using car for work yet, it sat for a week on our drive. went to it and only 4 lights were lit!. made a few short journeys that week to a total of 11 miles since filling, then it sat on my drive for another 6 days. went to it again down to 3 lights ! . from 6 lights to 3 in less than 20 miles? .
no signs of theft (although the crafty gits might be). ie , no smell, no spillage stains on the paving, the locking cap intact.

so, are the tanks shaped in such a way that the top bit disappears but the middle bit will last a decent time? or.. is the sensor telling fibs?
i wish i'd waited for a while when filling and then slowly brimmed it so i know it was deffo full . maybe asda shut off very early? however it took 35 litres, on top of 2 lights already in so musta been pretty close.

any ideas or help appreciated. thanks in advance.

davec63 wrote: ↑Thu Jul 09, 2020 6:49 pm so, are the tanks shaped in such a way that the top bit disappears but the middle bit will last a decent time? or.. is the sensor telling fibs?

The fuel gauge and tank is very linear.

You don't get a jump in the levels other than the steps in the fuel gauge.

I have seen corrosion build up on the surface of the resistor that forms part of the fuel level sender. It's rare but it does happen.
If the car has been sitting for a while with fuel at a particular level, electrolysis can cause corrosion to build up at that point.
You can either clean the contacts or wait and see if the wiper contact on the sender wipes the contact clean again.
Either way I would put up with it until I could determine that it's going to fix itself or not.
By the way I have found most fuel level indications aren't linear...well...slightly exponential. It seems to take about 100Km (80miles?) before the first 'indicator' (dial or LED) goes but less than 50km for the last one to go. Most of the cars I have owned have about a 40 litre tank. I usually fill my cars when they get to 1/4 of a tank because that last 1/4 seems to go very quickly and I have been 'caught out'! I have owned a 1976 ford Cortina, 1984 mazda 626, 2X 1980's Mazda 323's, a 2002 Mitsubishi Lancer, 2004 Citroen C2, 2X 2003 Citroen C3's (LOL) and that last 1/4 of a tank goes much faster than the first 1/4 on all of them.
Not that any of that has anything to do with your problem, just an observation.

just a bit of an update. today i discovered the gauge is being a bit temperamental. (at least the theft worry has been removed). you won't believe it tho . . went for a drive out with the mrs up and around kendal way . when i noticed the trip meter had reached 60 miles (still showing 3 bars btw) i decided to fill up again, just to see how much fuel was missing or had been used. the fun begins... initially parked on the wrong side of the pump for the tank, but, seeing as it was quiet i moved over rather than stay and hope the hose reached.
it was at this point... the stop, then almost instant restart to move caused the fuel gauge to read SIX bars !! i only noticed after the move to the other pump . well... if ever footage from morrisons in kendal gets released of a pale blue c3 hopping around the place then driving off without purchasing anything....that'll be us
just thought i'd share this lol. we'll see how the gauge goes in the future