Raikkonen did more testing Thursday in Rockingham, N.C., after two days earlier this week at Gresham Motorsports Park in Georgia attended by Busch.
All this comes less than a week after Raikkonen signed with Kyle Busch Motorsports to make his NASCAR debut next month in a Trucks Series race at Charlotte Motor Speedway.
"The test went really well at Gresham. Kimi is a race car driver, so he does know how to drive race cars and drive them well," Busch said Thursday. "I got to see it firsthand, and then got to get in the truck myself and run some laps just to kind of see where I compared to where he was at on tires and stuff, and we ran identical lap times. So he's right there, he knows what he's doing."
Raikkonen spent nine years in Formula One, driving for Sauber, McLaren and Ferrari. He had 18 wins, 62 podium finishes and won the 2007 Formula One championship. He currently races for the World Rally Car Championship with his own team, ICE 1 Racing.
"He did good again," Busch said. "Ran a lot better than we expected to run there, a lot faster than we expected to run there. I'm real happy with it."
Juan Pablo Montoya, the first Formula One driver to make the Chase for the NASCAR Sprint Cup, was a teammate of Raikkonen's at McLaren. He said he wasn't surprised by the early progress in NASCAR.
"Kimi has a lot of talent," Montoya said. "He will be fine. If anybody could do it, it would be him."
The Finn will make his NASCAR debut on May 20 driving a KBM Toyota Tundra in the Trucks Series race at Charlotte. That race falls between rally races in Sardinia and Argentina.
Busch said Raikkonen has also expressed interest in running Sprint Cup and Nationwide races, but Busch said will be re-evaluated after the first race in the truck.
"As far as the Nationwide and Cup stuff goes, we haven't had any discussions on that," Busch said. "He has asked after the truck debut in Charlotte, which is what our first focus is, kind of evaluating and seeing where he's at and how he feels and what he wants to do."
For now, Raikkonen is expected to drive between three and five truck races with Busch and be responsible for sponsorship money. The schedule beyond Charlotte is undecided.
Raikkonen described as "a lot of fun" his testing at Gresham.
"I really had no expectations or anything to compare it to beforehand but at the end of the day, I am really looking forward to the first race," he said in a statement. "It seems like KBM is a top team and I am excited to work with them."