Many of the low cost car rental companies in Spain and Portugal now offer cheap car hire prices, but have a "collect full- return empty" fuel policy. This means you have to use a full tank of fuel during your rental. Many people don't read the Terms and Conditions when they book a rental vehicle, and get a nasty surprise when they collect the car and are charged for a tank of fuel.

I would recommend that anyone hiring a car in Europe reads the fuel policy before booking a rental car.