FONTANA, Calif.—Jimmie Johnson had enough time to think about how good his car was—and worry that he might blow it.

“I’m just happy that we all did our parts tonight and didn’t let this great opportunity slip away,” the two-time reigning NASCAR Sprint Cup champion said after an overpowering victory Sunday night at Auto Club Speedway.

Johnson’s third victory of the season served notice that he is not to be taken lightly with the start of the Chase for the championship only two weeks away.

“It was an awesome race car all night long,” Johnson said after leading...

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