Hope this is not off topic!!
There is a growing number of car hire firms working a fuel [problem]. You pick up the car with a full tank of fuel?? When you return it at end of hire the fuel tank should be near empty. At commencement of hire, they take money from your CC, equivalent to full tank, so if you return the car with some fuel in it, tough luck.
Now the [problem]. In some cases, they charge for more litres of fuel than the car manufacturer says tank can hold. They charge a higher price/litre than the local filling stations. (some do tell you about the higher price)
It may well be that dishonest customers have fousted this proceedure upon the hire companies, but there are other ways of dealing with this.
This caper can be dangerous, as many airports are bye motorways etc. Only a complete idiot would run the risk of running out of fuel on a major road especially when booking in for a plane, so in the main, the hire firm scores again. If you keep searching, you should eventually get a hire company that will agree to a "Full to Full" policy.
Where very cheap hires are advertised, be very careful of not only the fuel policy, but insurance. Some are now hiding excess waivers.