Alain Visser, Opel's executive director for European Marketing, announced the extension in an interview with Automobilwoche.
The reason, he said, is the "overwhelming" demand for the free, three-day test drive, which has led to long waiting lists at some of the participating dealers in 40 European countries.
In Germany, where the campaign is called "Germany does the Opel test," the automaker has already sold 1,000 of the 35,000 test-drive vehicles and collected the names and addresses of a half million potential customers.
Visser said he was particularly happy that about 60 percent of the test-drivers were coming from other brands, notably Ford and Volkswagen.
A "conquest rate" of about 30 percent is expected, he said, adding that the participants are significantly younger than anticipated, averaging about 40 years old.