The 22-year-old Formula 2 convert ended the first day of official IndyCar testing at the Thermal Club track in Palm Springs tenth of 27 runners.
Armstrong was 0.535s off pace-setter Colton Herta’s top time and 0.510s behind leading Ganassi driver Alex Palou.
Having tested at Sebring with Dale Coyne Racing last fall, it was Armstrong’s first day of running with the squad he’ll compete in all road and street courses with this season.
The New Zealander will step out of the car for the oval races, with two-time Indianapolis 500 winner Takuma Sato taking over.
Reflecting on “a cool day”, Armstrong said: “I enjoyed it from start to finish. We were very organised and we got through our programme that we wanted to do.
“At the beginning everyone waited a lot longer than what I was expecting. I think I was maybe the first one to go out and do a proper run.
“But straight away I felt confident with the car. Obviously my second day in an IndyCar, still learning the whole car, but this Ganassi car certainly gives me a lot of confidence.
“The way it moves is very natural, if that makes sense… Honestly, it's not massively dissimilar to what I'm used to in F2.
“I think there's a few things that I prefer about this car, suits me a bit more. The way it moves is very natural.
“I think there's potential to do good things.”
He added that he felt “it was easy to identify what I wanted to change” as “he ended up getting through a lot of test items”.
Armstrong will join 2021 champion Palou, six-time IndyCar champion Scott Dixon and 2022 Indy 500 winner Marcus Ericsson in the four-car Ganassi roster this season.
Asked how he would compare his driving style and data traces with his team-mates, Armstrong observed that he has a similar approach to Palou with his smooth steering inputs.
He also praised team stalwart Dixon for “being extremely helpful to me, not only today but just through the whole process”.
“Everyone gets along really well,” he said. “There's a good synergy inside the team, especially with the drivers.
“It's fun to trade stories with Marcus as well, he's obviously been there and done that in F1 in Europe. That's sort of a scene I'm more familiar with. Alex, as well, I raced in F3.
“I feel like it's a good group of guys who have a lot of experience. I can learn a lot from them.”
