The advice comes from leading comparison site Carrentals.co.uk, which has been told of a number of cases where customers were charged for issues such as a damaged clutch, which would have been built up over time.
Gareth Robinson, Managing Director of Carrentals.co.uk, says: "Most people will check over their rental car before driving off, but sometimes there can be problems that can't be found until the car is actually being driven. A clutch that has been damaged by frequent riding, brakes affected by aggressive usage over time - they're just some of the potential issues that can build up and result in one unlucky customer being unfairly charged. Our advice is that if you have any concerns about your car - be that the oil level or braking performance, contact your rental company straight away.
"Rental companies do regularly maintain their cars, but it's always possible for unseen damage to occur over time. Our focus is on ensuring our customers aren't financially hit for previous misuse."
Carrentals.co.uk Top Tips to Avoid 'Unfair Wear & Tear' Charges -
Shop around for the most comprehensive excess insurance cover before you travel - and make sure it covers things like tyre and windscreen damage.
On collection, check over your rental car carefully - take your time and highlight any concerns with your supplier.
While you may not be able to see things like a nail in a tyre or a clutch that is slipping, once you start driving the vehicle take your time to feel if everything's OK and if you have any concerns go back to the supplier and let them know straight away.
Again, once you're out on the road or in your destination, if something's not right with your car, don't wait until you drop it off to highlight the problem, contact the supplier and let them know.
Keep a record of your contact with your supplier, including phone calls and written communication to ensure you have all of the details in case you need to challenge any charges.
Gareth adds: "While many people now take out excess insurance cover, some elements such as damage to windscreens and tyres just aren't covered with many suppliers, despite them being the most likely to be damaged. Make sure you know what you're getting with your insurance cover, and if in any doubt about your car's performance report it to the supplier immediately."
The award winning Carrentals.co.uk compares cheap car hire deals from companies including Alamo, Auto Europe, Budget, Holiday Autos and Sixt, and has booked over 650,000 rentals to date. Offering car hire in Portugal, including car hire in Faro, as well as Spain, France and over 15,000 locations worldwide, for more information go to http://www.insurance4carrental.com/Carrrentals.htm