Inspectors from the city Department of Transport, operating incognito, checked all taxis plying to the airport on Monday.
One of them, registered in Ho Chi Minh City, was told to drive from the international terminal to Tran Quoc Hoan Street and return. The fare for the trip normally costs VND54,000 (US$2.30), but this meter showed VND540,000.
The other taxi is registered in neighboring Dong Nai Province.
The scam worked through the installation of a switch under the gearshift, which increased the fare automatically by VND3,000 every time the driver changed gears.
Each driver was fined VND700,000 and their companies, Sai Gon Tourist and Saigon Taxi, were fined VND11 million each.
The inspectors also impounded the taxis for two months.
Tan Son Nhat is the busiest airport in Vietnam, receiving 60,000-70,000 passengers a day on average.