The City Department of Investigation says investigators posed as passengers to take 20 taxi trips, and all were charged at the right rates.
Last September, 59 drivers were charged with ripping off thousands of riders by repeatedly setting meters at a higher suburban rate when they actually were in the city. Officials have said the average passenger overpaid by $5.
Authorities say 49 drivers have been convicted or pleaded guilty so far.
The city's roughly 13,000 yellow cabs now have equipment that alerts passengers if the higher rate is being charged.