INDIANAPOLIS (AP) — Tony Stewart passed up a chance to race for Roger Penske in the Indianapolis 500.

The three-time NASCAR champion said Wednesday at the International Motorsports Industry Show that he wouldn't attempt to race in both the Indy 500 and NASCAR's Coca-Cola 600 on the same day.

"As much as I would like to do it, we just don't have the time to do it proper," Stewart said. "The IRL is so competitive now, you're not going to just show up like drivers used to do in the past and be competitive. These guys don't leave anything on the table there. You're not going to stroll into the Indy 500 with these guys who race every week and be as competitive as they are."

Penske extended the invitation last week in Las Vegas while accepting the Sprint Cup trophy. Later, when given the chance to shrug it off as a lighthearted moment with Stewart, Penske insisted he was very serious about fielding a car for the NASCAR owner-driver.

"It was a very humbling comment and offer that he made, for sure," said Stewart, booed by fans when he made the announcement. "I'm very humbled by the fact that he offered it to us. It's a great opportunity, but it's very hard when you're running three Cup teams right now and the obligations that we have making sure that we're doing the right things, we have a lot of people that depend on us. ... It's kind of like being at Thanksgiving dinner. My plate is finally full. I don't know if I can add anymore to what I have right now."

He left open the possibility of racing for Penske at some point.

"I'm not going to do it this year, but there's nothing saying that down the road that I may not take him up on that same offer," Stewart said.

There was a push to get Stewart to race in Indianapolis since Penske made the offer, with Indianapolis Motor Speedway starting an online petition for fans to encourage the driver to accept the ride.

Stewart has raced five times in the Indy 500, starting from the pole as a rookie in 1996 and leading 44 laps before his engine failed. His highest finish was fifth in 1997.

The Indiana native twice ran both the Indy 500 and Coca-Cola 600 on the same day. In 1999, he was ninth at Indy and fourth at Charlotte, and in 2001, he was sixth at Indy and third at Charlotte.

Penske-owned cars have won the Indianapolis 500 15 times.

Stewart keeps a close on the IndyCar series and the 500.

"I don't miss a thing that happens here," Stewart said. "I'm watching everything what's going on. It's definitely something we pay attention to."

Brad Keselowski, the NASCAR Sprint Cup champion for Penske, said he'd like to try a double himself.

"Tony's a Chevy driver. If Ford had an IndyCar presence, I hope Roger would talk to me," Keselowski tweeted. "Who wouldn't want to do it?"