I always make a habit of resetting the mileage trip meter to zero on filling the tank, that way I know roughly the miles I have done whatever happens. At 58mpg and knowing both the tank capacity and how much fuel was purchased, it’s easy enough to work out how much fuel has been used and fill up when required. 
I don’t know the modern cars any more, computers on wheels nowadays, too much like my laptop. At a guess it’s possibly a faulty sensor or it could be a loose connection.
Probably a problem with the sender in the fuel tank Mups. Sometimes these are easy to get at through the boot floor, and sometimes it means removing the fuel tank, but I’ve no idea on cost you would have to ask at the garage.
However, if you aren’t bothered about it too much you could just fill the tank each time you put petrol in and keep a note of the mileage using the tripometer to give you an idea how much you have used, a couple of times doing this after around two hundred miles should give you a good idea of your average mpg, if you don’t know already.
I agree, the sender sounds like the problem or the float is stuck inside the tank. Not normally a difficult job to fit a new one.
In one of my old cars, my petrol gauge would actual go UP whilst I was driving. That turned out to be a loose wire from the sender, touching the bodywork.
Most cars manufactured in the past 20 or so years have a delayed fuel gauge to stop any needle bounce from the fuel slopping around the tank while driving. If yours does not move after say around 3 to 5 mins after filling, then I would suggest your sender unit in the tank needs removing and checking or replacing after the wiring/grounding had been checked.
best let a garage replace the sender, easy enough to do the only problem is making sure the gasket is sealing the sender to the tank properly or you will get petrol fumes in the car. Unit should cost around £25/30 plus fitting so not an over expensive item to replace.
Done that job a few times in the past it is the petol fumes that get you
What fiat model is it mups…?
It’s a little Fiat Panda Easy, Primus.
Thank you for all your suggestions folks, much appreciated.
Just one more thing though, if it is the Sender - what the 'eck is a Sender?
Never heard of it. 
P.S. Nipped out to the local shop just before it got dark tonight and I see the guage has moved to where it should be.
Wonder why it’s taking about 12 hours to do that though?
That’s my technique too I know I can get about 700km out of a tank. You soon get into the habit, I use “A” trip meter for individual trip distance, “B” for fuel consumption.
Having said that the fuel gauge is actually pretty accurate
That sounds like the float is sticking, it’s located in the fuel tank, if it’s run a bit low and not used, it may have stuck down, and not risen until the pressure of the fuel in the tank has pushed it up, ( think cistern ball clock) , if it keeps happening, you need to get it looked at
I also use both the petrol gauge and mileage trip so that when it’s on ‘half’ I fill the tank, that way
I always have at a minimum half a tank of fuel, if for some reason I cannot get to the petrol station. Old habits die hard! 
Cars these days are like laptops on wheels, the onboard computer can be used to check fuel consumption, which I do occasionally. The VW Golf 1.6 diesel I drive returns 58-61mpg (93-98km to 4.5 litres), hopefully conversion is correct, which I reckon is brilliant. 
Fiat Mupster. :-p
My car is not that sophisticated the next model up has all that gear but I decided I could find out the temperature outside by opening the window so I just keep a record of fuel on a spreadsheet.
I don’t get anything like the economy that you do but it is a 3 litre turbo diesel driving a 2 tonne vehicle so about 10L/100km is not bad especially as those figures are towing a camper too.
BTW I used to think the “litres per 100km” was a daft measure but I must admit it is actually very useful and like it now.
Thats what I do too, Baz. I fill up when it gets down to the half way mark.
That makes sense Primus, thank you.
I think its due for a service next month, so I’ll mention it to them if it’s still happening then.
I assume it’s safe to leave for a few weeks?
Oh you shush.
These ‘sophisticated’ cars are OK, until they go wrong then they can be very expensive to repair.
I still haven’t even worked out some of the functions and I’ve had the car for seven years!
My other car, a Scimitar GTE, has a 2.8 V6 Ford petrol engine, around town that does about 20mpg, on a journey I might get 32mpg so rather thirsty. The consolation though is performance and rarity in style as perhaps a ‘one day classic’. I haven’t used it for some while now though, another project to get ‘sorted’!
I have yet to go completely metric, although on most things it does make more sense. Those figures I quoted seemed very strange. Perhaps I’ve been using imperial measures for too long? 
Better than taking chances I reckon and it’s a habit that is difficult to get out of too,
even with my car being like a laptop computer on wheels!
Mups, how much fuel did you put in?, sometimes if you only put a few quids worth in it can take the fuel gauge a while to register on some cars, if it appears to be working now, this could be the case, it will be ok and you can drive it, just be careful you don’t run out of fuel, I know from experience with mis reading fuel gauges…
I don’t drive every day anymore, since I retired I find there is no need to. I don’t do motorway driving either, so just for poddling about locally I find a tenner a week will usually do.
If I get really excited and venture further, £15 would cover it. 
Thanks ever so much for the reassurance though, Primus.
When I am travelling I fill up every day, personally when ever I get fuel I always fill the tank.
I recall that when I was in the UK in the late 1980s for my old man’s funeral I drove my mother from Folkestone to Glasgow (it was the night of the Lockerbie crash). She insisted that we stopped at every servo on the motorways to fill up with fuel, it drove me up the wall. In the end I just refused to stop until the tank was three quarters empty it is not as if the UK has any shortage of fuel stations.
