Castiza,
You have found the answer: You live in Madrid (so you know the city) and you probably speak Spanish. Under those circumstances, there are few chances to get rip off by a taxi driver.
However, if you don't know the city or don't speak Spanish, you are likely to be ripped off by a taxi driver.
From my experience at Hostal Chelo during 14 years, most of our non-Spanish speakers customers got ripped off when they took a taxi in Madrid.
I always ask my customers how much they were charged and very few of them find a honest taxi driver. And, even then, the fare was always more expensive than the equivalent offered by Aerocity.
That's why we started to work with them a few months ago. We were fed up with taxi drivers ripping off our customers. A remember an old couple who were charged 5000 pesetas from Colón bus station to Hortaleza street.
Tonight, we have two English women who were charged 2600 pesetas from the airport to Hostal Chelo. This is a normal fare but with Aerocity they would have paid only 1900 pesetas. Of course, they have asked me to book Aerocity for them when they leave next Saturday.
I wish taxi drivers were honest - as they are in England - but they aren't here in Spain. Fortunately, there are plans to install new meters with printers (so that you always get an official receipt). Also, these meters will be monitored by a GPS system and this should avoid most rip offs.
By the way, some taxi drivers have also tried to divert our customers to other "better" hostels. The cheat begins by telling the customers the place they are going to isn't a good hostel. Then, he would say he knew a good one and he would persuade them to go there. Of course, that place is so "good" because they get a commission for the customers he brings to them.
Antonio
PS: The highest rip off I've heard of was 30000 pesetas for a ride from the airport to the city centre. The unlucky victims were two Japanese girls.
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The best tips from your favourite hostal in Madrid.
Hostal Chelo at
http://www.chelo.com