Alonso may have failed in his bid to win the Indianapolis 500 as a rookie in May -- his engine blew up with 21 laps to go
of the 200-lap race -- but he came away with a huge admiration for American motorsport fans.
"It's a different culture, a different way of understanding motorsport or sport in general -- much more open, much more friendly to all of us," Alonso told CNN's The Circuit.
"I imagine if the (Indy 500) was in Spain and some US people came here our favorites would be the Spanish sportsmen and we would cheer for them massively," he added, speaking at his karting circuit in his home town of Oviedo in northern Spain.
"It was the kind of feeling that the sport is beyond nationalities or anything like that. All of us there were heroes for all the people in the grandstands. They support all of us in the same way and they really enjoyed the race, so that was a good lesson we all learned."
F1 and Indy legend Mario Andretti, whose son Michael spearheaded Alonso's Indy 500 campaign with the McLaren-Honda-Andretti team, says the Spaniard's one-off switch to Indy car was great for motorsport.
"I've seen clearly that he started appreciating the demonstration of support from the fans," Andretti told CNN. "They showed their appreciation of his effort and he saw the value of that," the 1978 F1 world champion added.
"In Montreal (at the Canadian Grand Prix), when he dropped out, he went right into the fans with his helmet on ... and that's what it's all about. That's what fuels the sport. That's what the fans like.
"So he brought something to Indy that I think was very valuable and important. He was received with open arms by everyone. It was a good thing for the sport -- good for Indy Car and F1, quite honestly."
Alonso arrives in Austin for round 17 of 20 in the F1 World Championship with his future still undecided.
The two-time world champion has yet to commit to McLaren for the 2018 season although since the British team switched engine suppliers
from Honda to Renault, it is widely expected that Alonso will continue with the team.
The United States Grand Prix takes place at the Circuit of the Americas
on Sunday October 22.