Rubens Barrichello is the only driver on the planet to have raced in a 2011 F1 Grand Prix as well as having driven the new-specification 2012 Dallara DW12 IndyCar and the V6 Chevrolet turbo engine, giving him a unique position to compare and contrast the experience in the two series.
So: what had caught his attention so far?
"The first surprise was the fact that I didn't have the warmers on the tyre. You know, I went out, it's a different technique to drive," he admitted straight away. "When I went out, Jesus, I didn't have the temperature on the tire, and I almost spun. So that's a new thing for me."
It's just one of the little tricks he's going to have to pick up from his 'brother' Tony Kanaan as he adjusts to his new challenge. "Tony is very, very good at that. He goes out, and he feels like he has new tyres all the time," said Barrichello admiringly.
"The other day I was testing in Sonoma and left the pits, turned the pit limiter off - and I almost revved the engine higher than what it should rev because my mind was still set up for my 19 years that I was in F1," he continued. "So different revs, different everything."
Happily there were some areas that were much more comfortable for him during his first two test sessions with his new team, KV Racing Technology.
"The gearbox was quite good, the engine was nice and drivable. The brakes were fine, as I had carbon brakes my whole life, and now it's there in IndyCars all the way," he said. "The steering wheel is a little bit heavier, so I have to get used to that. But apart from that, I mean, the car gives you great feedback on the high speed corners and braking stability.
"It's still not natural for me because I only had a few tests with Tony's car," he pointed out. "I have another two days at Sebring and one day at Barber, and I don't know if I'm going to be well prepared for the first race, but what I can promise is that I will push to the very limits," he said, adding: "It's just a different machine, and I need more time in it. But I had a lot of fun driving."
Of course, it's not just the difference in technology that Rubens will have to adjust to: it's also a very different sort of culture, including much more up-close and personal interaction with the fans in the US than the arms-length distance than F1 fans are kept at during Grand Prix weekends.
"I like the fact that we're going to be closer," he said, genuinely. "At Sonoma we had like a thousand fans on the track, 1,500 there, and it was great. It was a great reception. It was a good mood all over ... It's well organized. People don't go across the board, and they just know where to stand.