During the 2022 Music City Grand Prix, IndyCar fans might have noticed something new. Instead of the usual tire choices of the red and black tire compound duo from Firestone, the weekend saw a green sidewall tire replacing the reds. Made from guayule, or a desert shrub sourced in the United States and Mexico, these sustainable tires have been a — which means their debut went off without a hitch.

In a conversation with Cara Krstolic, Firestone’s chief engineer in the IndyCar realm, ahead of the race, I learned that the whole goal of the guayule tire was to create a compound that was exactly the same as the red sidewall tire it would be replacing — and that process started ages ago.

“When we started, we asked, here’s this material, how is it chemically similar to our other rubber that we use? There are different grades of rubber, so we need to make sure we get the same grade of rubber so it would be chemically equivalent,” Krstolic said. That’s to ensure that the new rubber substance can serve as an easy one-to-one replacement for the outgoing hevea rubber. “Once we got to the point where we thought the guayule could be an equivalent compound, we built a tire and put it through all the standard testing we would normally do, plus a little more on just the tire itself.”

After durability testing that included long-distance running and running at higher speeds than you’d ever find in an IndyCar, Firestone found that the green tires would essentially perform exactly like the red tires.

Krstolic assured me that there would be no surprises or quirks when it came to the on-track performance of the green tires. Firestone would be watching the tires with interest, but the Nashville race was by no means a proving ground. The hard work had already been done.

And according to the drivers, the green tires were fine. Team Penske’s Josef Newgarden said they “felt like a normal red in a lot of ways,” which was exactly Firestone’s goal.

When I asked about the future of the green tires, Krstolic noted that the Nashville running was just a preliminary outing; any future implementation of the tires would come at a later date — and that includes on-track or on our road cars. So for now, Firestone can rest easy.

“Well get some data back and then we make some decisions on where to use it next year,” Krstolic said. “We’re still working on the growing process [of guayule], and we’re still working on commercialization. In order to make this sustainable tire truly sustainable, we need to make sure there are other people that can grow the product. Then we’ll see where we’re going to use it next year.”