Hidden car hire fuel charges exposedCheapest deal can become most expensive

Holiday car hire deals may look cheap when you book them, but Which? members have been telling us that there are some nasty hidden charges to look out for when you collect your car for some driving in the sun. The most complaints came about car hire companies in Spain that charge for a full tank of petrol when you collect the car, potentially leaving you out of pocket if you don't drive far enough to use it all.

So we sent a researcher to uncover exactly what was going on.

We've received lots of complaints from readers about the cost of fuel at Malaga. So I am here, Kathryn Liston, here in Malaga, to investigate and find out just how much people are being charged. I've just picked up this car from Advantage at Malaga and been charged 58 euros, actually 59 euros for a full tank of petrol.

That's twice as much as I've paid for the rental of the car. And I'm only here for 4 days so I'll never make it.

What really amazes me over here is that you always have to ask what they have taken off the credit card? You're never given a slip to sign that says 100 euros or 60 euros, it's really, really confusing.

Day two at Malaga Airport. We're back here to pick up a car. This time we hired Goldcar, and let's see what their policy is. Just paid just over 54 euros for a tank full of petrol, which is showing full, and I have to use it in four days. In a little car like this, that might be impossible. We're driving through Torremolinos, one of the busy roads going down from Malaga.

Most people would be going down through Torremolinos down Marbella, Puerto Banus, Fuengirola. So, it's not that far really, so you're not going to use too much petrol. So, how are we  going to use full this tank in four days, I have no idea. What makes me laugh about all these car hire companies is not one of them has explained clearly to me what I'm paying when I'm handing my credit card over for.  No, it's only when I've questioned them, what am I paying for, that they mention the fuel.

Just driven 150 kilometres, which is about about two and a half hours of driving today and I've used about an eighth of of a tank of petrol. It's going to go some to use the rest of it. we're off to Mijas today just to see how much petrol we use today. I reckon yesterday in the small little Fiat we did about a hundred and nineteen kilometres in about two hours.

So, I reckon you'd have to do nearly 200 kilometres a day in that little car just to use a tank. If you're only here for four days. So, that's not possible unless you're going to spend most of your holiday or your trip driving around. I don't see how you would ever use this full tank of petrol. We've been doing what any normal tourist would have been doing.

One long trip down to Marbella in to Torremolinos, out to Malaga. Now we're just having a day out. And still the petrol gauge is not even down, so we haven't even used a quarter of a tank of petrol.  It's absolutely ridiculous. Right, just back from Granada, 300 kilometres, 3 hours driving, and we're nearly on half a tank of petrol.

So you really do have to do some motoring to get rid of the petrol. Right, here we are again at Malaga just dropped off the car, used about a quarter a tank of petrol. Asked for a refund. They said no refund. They were quite adamant about that. 

Our researcher has shown that these unavoidable fuel charges can triple the price of a rental, turning what looked like a good deal into a very expensive one. Consumers should not be put in to the position where booking a car rental commits them to paying an unavoidable charge with no refunds.

All charges should be made up front at the time the booking is made so consumers can make the right choice and compare prices easily.