Even with the taxis using meters I got ripped off three times. Once was by not being familiar with the city and I took a taxi from the train station to the Hotel--which was 5 blocks away! It still came to 5 Euros. Another time I got lost 2 blocks from the hotel and we drove around for a while longer than necessary. The last time, my teenaged daughter told off the driver in a mixture of Italian and Spanish that he had passed the street.
My friends and I were tired and unknowingly victims of this two times while In Rome. Listen carefully. We were warned about pickpockets and gypsies. They were not a problem because we knew what to watch out for and were very careful. It's the taxi cabs that you flag down who will rip you off!!!! You have your luggage and you are tired. The cab driver drops you off at your location. He says it costs 20 Euros. You give him two ten dollar euros. You turn your back to remove your luggage and they say you only gave them 2 5's. It works on you because you may be unfamiliar with the money there and you are also distracted. We all thought we were so smart to everything in Italy. We read all the books and we were still a victim of this twice. Maybe they target groups of women, who knows. This is a good one and they will get you if you are not careful. Try to get your taxi at a hotel. Ask at the front desk how much this should cost. Have one person pay and count out the money with him and another person as a witness. Also, look at the meter when he shuts it. Very important!!! Hope they never get you like this!!!
Again, taxis are problem. We didn't want to take the car to the city center since all the huge amounts of one-way streets. What the city center is good for, is a splendid opportunity to practise your u-turn capabilities. Therefore, we usually drove our car to the airport since we had to pick people from there and then took the taxi from there to city center and back. For the three people, the price varied from 9 euros to 20 euros and we didn't understood where the price difference came up from. We made quite a few trips and the price tag was always bit of guessing.