The Indian drove for the Starbeast Motorsports team, which is affiliated with NASCAR Sprint Cup squad JTG Daugherty Racing, on the opening day of the test.
"It's a whole new ball game out there," Karthikeyan was quoted as saying by the Times of India.
"I was lucky enough to be invited for a few sessions. NASCAR racing is pure, more basic with its oval tracks and inclined speedways.
"It's a totally new experience driving heavier cars with spotters speaking to you all the time."
The 32-year-old has primarily raced in A1GP since his F1 career ended with a Williams testing stint two years ago, although he also contested several Le Mans Series races for the Kolles Audi team this season.
He said had planned to continue with A1GP prior to the series' collapse, and will stick with the nation versus nation championship if it is rescued.
"It has thrown things open right now," Karthikeyan admitted. "The financial downturn caused havoc. If A1GP survives I will be there next year. It is a great championship with a lot of competition.
"However, we will have to sit down and discuss the future course of action if things remain the way they are."
Patrick eyes summer NASCAR racesAdriano offers on table
