We recently hired a car from SIXT through cartrawler and it was the first time (we have hired 4 times this year) that we had to return the car with an empty tank.
On our return to the UK we received the invoice and they had charged us 94,86 euros for the tank of fuel, when it had cost us 43 euros to fill up the small Golf car.
Needless to say we will not hire again from a company that insists on an empty tank on return.
Ozzie