The yellow taxi service is excellent in Rio. I have never had a problem with a dishonest driver. You pay only what is on the meter - no tipping is expected (although you can round up to the next real).
If you need a taxi to the airport you can schedule one with Libertaxi (registered clients get a discount of 20% off on fares over 15 reais). If you meet a driver you particularly like you can negotiate with him for a rate for a full day of driving around the city, or even up to Petropolis (the former summer capital) and back.
Rua Cirne Maia, 100 Cachambi
Tel: 2105 0500
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