In theory it's much cheaper to shop around and book a hire car in advance than when you arrive at a foreign airport – but confusing and unclear charges make it virtually impossible to identify the best deal.
From excess charges, excess waivers, super collision waivers, damage collision waivers, fuel charges and fees for sat-navs and child seats, comparing car hire deals can drive you mad.
In what sounds like the job from hell, Which? Travel researchers went through the booking process 120 times on 10 of the most popular car hire rental websites, and said the total price was not clear for more than half the bookings.
Of the researchers who had a compulsory fuel charge to pay, 75% said they did not know how much it would be. Nearly half (44%) the researchers didn't know the cost of the optional excess waiver to reduce the amount they'd have to pay if the car was damaged – even though it can add more than £100 to a week's rental.
One of the biggest issues holidaymakers have with car hire firms is the level of excess they have to pay if the car is damaged while it's in their possession.
Forget the £150 or so that's the norm on a car insurance policy – excesses on hire cars can be eye-watering. £500 or £600 is standard but you can fork out more than £1,000 on some models.
And don't expect the car hire firms to be particularly forthcoming about the level of excess either; you'll need to check the small print in many cases.
Collision damage waivers
To counteract high excess charges car hire firms offer insurance to reduce the amount you'd have to pay in the event of a claim. Often they'll only flog you this cover when you arrive at the rental desk to collect your car.
This insurance is normally called "collision damage waiver" or its big brother "super collision damage waiver" or simply "damage excess insurance".
You might think from the name this cover would waive your liability in the event of an accident – but think again.
In many cases you'll still have to fork out an excess in the event of an accident. Typical small print on policies states that this insurance won't cover damage to certain parts of the car such as tyres or windscreen, or lost keys.
As well as not being as comprehensive as you might expect, buying excess car hire insurance from a car hire company can significantly add to a traveller's costs.
Research from icarhireinsurance.com found that excess insurance for a week's car hire in Nice, France, could be as much as £128 from Hertz, adding 66% to the cost of the rental (£195). Even then the cover excludes the car's tyres and windscreen. But without the cover you'd face forking out up to £1,163 excess in the event of a claim.
Buy separate cover
A top tip for travellers is to buy stand-alone excess car hire insurance before they set off. Companies such as icarhireinsurance.com and insurance4carhire.com offer policies that reduce your liability to zero and also cover parts of the car other policies don't include.
Annual policies cost about £40 and cover an unlimited number of rentals per year.
Different car hire firms have different policies regarding fuel so make sure you know what you're agreeing to before you drive away.
Some car hire firms expect you to return the car with a full tank of fuel; if you don't they'll charge you well over the odds to fill the tank up. So before you leave the airport make sure you know where the nearest petrol station is so you can fill up before you return the vehicle.
And, whatever you do, check whether the car runs on petrol or diesel. Fill it up with the wrong type of fuel and you'll be on the receiving end of the bill for the damage.
Car hire contracts typically involve extra costs for things such as sat-navs and child seats. In many cases it's cheaper to simply bring your own.
It's worth checking the car hire firm's policy on drivers' ages too. Some car hire firms won't hire to drivers under the age of 21 or 25 or, if they do, they'll bump up the cost.