Max Chilton admits the friendliness of the IndyCar paddock is a culture shock following his time in Formula 1.
The former Marussia F1 driver joins Chip Ganassi Racing in the series this year, after contesting Indy Lights in 2015 alongside Nissan LMP1 duties.
He has now completed two tests with the team, with the season to start at St Petersburg next month.
"It's quite eerie how welcoming everyone is," Chilton said.
"I understand why the teams are welcoming, but the drivers, I'm not used to that.
"The drivers get along; there might be a couple you have as sort of mates.
"Everyone in IndyCar, even with Indy Lights, the whole sort of Road to Indy process is so welcoming.
"All the drivers seem to be best mates and they don't seem to have other friends.
"Their best friends are I guess their rivals on the track.
"That's sort of taken me a while to get my head around."
Chilton found more familiar territory inside Ganassi's Chevrolet-powered Dallara, and the similarities with F1 tyre management during his first test at Sonoma.
"They perform well in the corners, better than I expected," he said.
"I got up to speed pretty quickly. In the afternoon we concentrated on not doing a full race distance but we did fuel stints, 25 laps at a time.
"It was good to sort of see how the tyres wore down throughout the run.
"It's very much what I'm used to in F1.
"The tyre degradation is pretty high, but it's manageable as long as you drive sensibly, control the wheel spin, you can find ways of extending the life of the tyre."
ANOTHER CHECKPOINT PASSED
Following his maiden run at Sonoma, Chilton tested at Fontana Speedway, his first laps in an IndyCar on an oval.
Team-mate Tony Kanaan was on hand to help, along with multiple champion Dario Franchitti, in his Ganassi advisory role.
"TK got in the car and did an eight-lap run," Chilton said.
"He was flat out pretty quickly. I stood up in the stands with Dario Franchitti and a spotter just watching how it seemed.
"Fontana's got five different sort of lanes. Was just keeping an eye on what lane he was staying on throughout the lap. So I learnt that way.
"In the first run, I don't think you can ever really feel comfortable on an oval. I know I got comfortable quite quickly.
"It was a good way for me to learn without other cars around and just built up to it as the day went down.
"As oval speedway first days go, it was a good day."