As I used to own a certain Station, Will not mention, I will say this, due to the structure of ordering and the way it has changed over the years you might think you are filling up @ certain station but you are not getting their petrol. Even as a franchise you may only buy from that franchise but what happens if you are out of stock and the franchise cannot deliver. Well there are people you can call that will deliver for you @ a cheaper price than what you buy at the franchise from. Yes if the franchise catches, you may be kicked of site but they have to catch you and also prove that it is not their petrol in your underground tanks.
So you think it you are filling up a X and getting X product.
As for over fill My gets had a 45 lt tank and I filled up 48 litres regularly. What it states is the fuel amount but your tank holds on avg 10% more than you think due to reserve, pipes etc. Thus a 80 lt tanks stated may actually hold 90L, depends on how low you drive it.
There could be a problem with Fuel pumps calibration, but this is easy to check.
If you think you are being short changed, every fuel pump has a mechanical reading(small number like old school odometers) Check that reading as you start to fill up and just make sure that as that reads per litre it = what is on the pump.
Also never, accept a garage to take an imprint of your Garage cart. If they do be afraid, as the only reason they do that is if there is something funny going on. If an petrol attendant puts in wrong KM INSIST on REDOING the transaction, but be aware the banks implemented a system some years back that you cannot use the same garage card at the same station within an hour and that you cannot use that garage card within an hour of last use, regardless of the station.
If you think your card has been used CALL THE BANK, they contact the service station and it is up to the service station to prove the validity of the transaction within 30 days or they reverse the transaction.
My rules when I fill up as an ex owner. I get out of my car, or watch it the mirror that the nozzle actually goes into my cars tank. I check mechanical pump readings and make sure they match electronic on the face of the pump. If they check oil I GET OUT AND MAKE, SURE THE OIL CAP IS REPLACED PROPERLY. If they pump my tyres I make sure, they blow some air on the paving stones to c if there is water in the compressor. If I notice moisture, I do not pump my wheels.
If you have people using a company Garage card make sure you check the OIL usage. Oil = R26/30 at some stations. Funny so does a pack of Smokes or a coke, chips and a bar one.
Cannot think of anything else @ the moment but if I do will post it. I can share some scary stories on Garage cards and how to use and abuse them.
Simple rule. TRUST NO ONE.