This correspondent has often commented on the problems of car rental. Here's another: I picked up a car recently to be told, as expected, that it should be returned with a full tank to avoid a premium fuel charge. The gauge showed full as I drove away but dipped rapidly. Subsequent observation indicated that, for the car in question, it hovers around the full marker for maybe 100 - 150 kilometres.
I'm not suggesting any deliberate attempt to chisel extra revenue - and even if I though there had been it would be impossible to prove. But the lesson, I suppose, is that you should keep an eye on consumption and if you think you have been provided with less than a full tank to start with, don't be too conscientious about refuelling at the very last service station before returning the car.
Of course, the problem would be solved if car hire firms simply refuelled at normal petrol/diesel prices - or built a small premium into their rates for any inconvenience. But despite improvements in recent years, simplicity is still something they struggle with