Ask the experts: technology expert Donald Strachan offers advice on finding the best car-hire deals.

Everything cheap seems to be on a website these days. Where, then, should I look online for the cheapest car-hire deals?

Donald Strachan replies

To discover which websites find the cheapest prices, I checked prices returned by eight leading meta-search engines and car-hire brokers for 20 representative trips. I spread pickup points for my compact car around the world (most in Europe or North America), varied rental length from four days to three weeks, and chose hire periods between now and Christmas, with most over the summer. The most best prices came from TravelSupermarket.com, but it was beaten on the total cost of a basket of 18 rentals by Comparecarhire.co.uk and Carrentals.co.uk. Less than £30 in total over 18 rentals separated the three. In fact, there was little to separate the total price returned by those three, Kayak.co.uk and Carhiresearch.co.uk. Cheaper prices were also returned for one rental each by Rhinocarhire.com and Netflights.com.

Your best bet, then, is to bookmark a couple of these sites. I like a clean design and for my search results to be returned quickly, and Carrentals.co.uk and Comparecarhire.co.uk score highly on both those counts. Both also have good Android and iPhone apps. Rhinocarhire.com is another with an excellent iPhone app, and was the only site to find a car for every one of my 20 searches. Kayak s outstanding iPad app is quick and elegant for car-hire searching.

The test also emphasises the importance of shopping around. For example, for a two-week hire from San Francisco airport starting on July 22, I was quoted between £306 and £490 on the various sites.

Whichever search site you use, there are rules you can follow to make sure you find the cheapest price. Most importantly, book as early as you can. Rates can go up or down the later you leave it, says Phil Partridge at Rhinocarhire.com. But they are more likely to increase, especially in peak season. Kevin Currie at Holiday Autos says booking eight weeks ahead is the optimum in popular destinations such as Spain and France .

It s also worth hunting online for discounts. Rhinocarhire.com offers five per cent off if you like them on Facebook. You ll often find money off Holiday Autos (holidayautos.co.uk) on the voucher and cashback sites. There is six per cent off at Topcashback.co.uk as I write, though offers change constantly.

Remember that search sites generally only compare the base price. If you want child car seats or additional drivers, you ll have to delve deeper. It could easily be worth accepting a higher base price, for example, if that price includes one additional driver.

And, while most of the major players offer inclusive insurance coverage, check the policy excess. Typically this is between £500 and £1,600, and an excess waiver costs around £5 per day from most competitive suppliers. If you hire a car more than once a year, it s worth taking out an annual excess insurance policy from Insurance4carhire.com, which costs £49 for a policy across Europe, £65 worldwide.

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About Donald Strachan

Donald is a specialist in technology and new media.

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