Search the forums. there's a thread by batang_raon14 regarding this.

Certain first generation Optras had a drinking problem. There was no recall, but Chevrolet Balintawak offered a silent warranty service (computer reprogramming) to correct this. (Which they should have made public knowledge, as rumors of the Optra's gas-guzzling certainly hurt sales badly). The second-generation (facelifted) Optra had this corrected at the start. From the official DOE fuel-economy tests and from third-party (magazine) tests, the new Optras are apparently of at least average or above average fuel-efficiency.