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Three-time Indianapolis 500 winner Helio Castroneves could remain a full-time IndyCar driver with Penske in 2018, if the team sticks to running four cars after all.

Mounting speculation suggests that, contrary to previous rumours, Team Penske will continue to run four cars in the 2018 IndyCar Series with an unchanged driver line-up.

It had been thought likely to slim down to three full-time cars for Simon Pagenaud, Josef Newgarden and Will Power, with Castroneves set to move to Roger Penske's IMSA venture.

Penske will run a pair of Penske-run Acura ARX-05s in the IMSA SportsCar Championship season, with Juan Pablo Montoya and Dane Cameron confirmed in one car and Castroneves tipped to lead the other entry.

Castroneves was known to be unhappy at the prospect, despite being offered 'one-off' drives at the Brickyard each May.

Several trusted sources, one a longstanding and senior Penske member, have informed Autosport that the team's engineers and crew chiefs have been told it will continue as a four-car IndyCar squad in 2018, while Chevrolet is also believed to be eager for the team not to be downsized.

In the summer Castroneves vowed to make it as difficult as possible for Penske to switch him from single-seaters to sportscars by winning this year's IndyCar title, and he enters this weekend's Sonoma finale 22 points behind championship leader and team-mate Newgarden.

Autosport has learned that Casontroneves and Penske will not be discussing the topic again until after this weekend's season finale.

Asked if this would be his last race at Sonoma, Castroneves said: "I hope not. I definitely want to come back.

"But the team, everybody, even myself, want to focus 100% on this last race [in the championship battle].

"It's mixed feelings for me, because I really have a chance - my chance - but at the same time, having Scott [Dixon] there, it throws a little bit of a curve ball in the team to accomplish the championship.

"But whatever happens in the future, trust me, I'll be back here at Sonoma."

Asked if that would be in an Indycar, Castroneves said: "Now that's another story!

"We don't know yet. Let's see. To be continued."

All nine of Chevrolet's wins in IndyCar this season have come from Penske drivers, and with Andretti Autosport electing to stick with Honda after a summer of talks between Michael Andretti and Chevy, senior GM personnel are understood to be even more keen to retain Penske's four proven race winners and title contenders.

One rival team owner, who did not wish to be named, said they "never saw any evidence" Penske was going to slim down to three cars.

The team owner added: "If he's going to stay at four, why wouldn't he keep Helio?

"He's the obvious choice, and let's face it, those four cars are all getting strong results, so why would you change the lineup when it's working so well?"