Tony Stewart believes he has an open-ended offer from Roger Penske to run the Indianapolis 500.

The three-time NASCAR champion turned down Penske's offer in December to drive one of his cars at Indianapolis Motor Speedway, but Stewart revealed Monday he told "The Captain" over dinner he'd like to revisit the offer down the road.

"It's a very flattering offer. I went back to Roger and actually had dinner with him in Indianapolis and told him he not necessarily backed me in a corner, but he made me a generous offer," Stewart said. "I made him a counteroffer that it's an open-ended invitation. So even though we haven't accepted this year, maybe some year down the road we may be able to accept it. He never said no, so as far as I'm concerned we have an open invitation."

Penske, a 15-time winner as a car owner of the Indy 500, first made his offer while accepting the Sprint Cup trophy and lauding Stewart, the 2011 champion.

"I know we took your spot this year," Penske said. "But you're a car owner, you're a race driver, you're a track promoter — how about doing the double at Indy this year? Are you available?"

Penske later insisted he was serious about wanting Stewart, an Indiana native and former IndyCar series champion, to drive for him in the 500.

Stewart, who grew up dreaming of winning the Indy 500, was silent about Penske's offer for several days. Then he turned Penske down.

He said Monday his role as co-owner of Stewart-Haas Racing and his responsibilities as a NASCAR driver and owner of several race teams and tracks left him unable to run the Indy 500. And, he said he wasn't certain he'd be competitive.

Stewart ran the 500 five times and finished a career-best fifth in 1997. Stewart twice attempted to run both the Indy 500 and NASCAR's Coca-Cola 600 on the same day: In 1999, he was ninth at Indy and fourth at Charlotte, and he was sixth at Indy and third at Charlotte in 2001.

He hasn't tried Indianapolis since.

"Indy car racing has progressed and the competition has gotten tighter and tighter there," he said. "I feel like (Penske's) the perfect team to go and do the month of May with, but I still feel like for me as a driver, I would need to start the year with that team and run through the month of May with that team to really feel like the first day of practice I was competitive."

Stewart this year will be fielding a Sprint Cup Series car for Danica Patrick, who is also sitting out the Indy 500 for a second consecutive year. She joked Monday that maybe Stewart could help her get a seat with Penske.

"I feel like Tony is my in now. Put in a good word for me, maybe? Maybe something in the future? Maybe the both of us can run it in the future?" she said before turning serious. "It's just something that didn't work out for this year, I think it's most important to focus on the Cup effort. I think it would be exciting and interesting to do it in the future, if it works out, it does. And if it doesn't I've had many great years at Indy and lots of great memories.

SHR competition director Greg Zipadelli was Stewart's crew chief for both of his double duty attempts, and he discouraged Patrick from pursuing a ride in this May's race. He understands how badly Stewart once wanted to win the Indy 500, but doesn't think Stewart struggled too much with the decision this time around.

"There's only a few people who can do it and be competitive, and there's only a few teams you'd want to go do it with that you know you are going to be competitive, so that's a pretty hard deal to pass up," Zipadelli said. "Tony had done it, so he knew how hard it was. I think he knows the Cup Series is more competitive than it was 10 years ago, 13 years ago, and so is Indy. So I think his decision was easier because he realized he has a lot at stake in where he's at right now.

"He has shown some maturity in a ton of areas and this is one of them. I'm sure it tempted him. Who wouldn't be flattered by the opportunity? But I think at the end of the day, he knows he made the right decision for right now in his career."