For the past two years, thousands of taxi drivers overcharged passengers a total of more than $8 million by switching the meter to double the rate, the Taxi and Limousine Commission said Friday afternoon.

Using G.P.S. technology installed in cabs, the commission discovered more than 1.8 million trips where passengers were charged the higher rate. The total amount of the overcharge was $8,330,155, or an average of $4.45 per trip, the agency said.

The agency said that drivers manually switched the meter from the standard rate of 40 cents per fifth of a mile to the 80-cents-per-fifth-of-a-mile rate that cabbies are allowed to charge in Westchester and Nassau Counties, but not in New York City.

To combat the overcharges, the commission said that within two weeks, a system would be installed in all taxis that would post an alert on the back-seat television screen when the meter is switched to the higher rate code. The alert would stay on the screen until the passenger acknowledged it.

The scam was primarily perpetrated by a small number of drivers, with 3,000 of them overcharging more than 100 times, the agency said.

While the 1.8 million overcharged trips is a large number, it represents only 0.5 percent of the 361 million taxi trips taken during the 26-month period the agency studied.

A single cabby was accused this month of overcharging passengers a total of $40,000.

The commission has referred the overcharges to the Department of Investigation.