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The couple that speeds together, stays together?!

NASCAR superstar Tony Stewart is punching the gas all the way to the alter ... he just got engaged to his drag racer girlfriend Leah Pruett and the diamond ring is MASSIVE!

Stewart confirmed the engagement on social media late Thursday night -- saying, "So excited to spend the rest of my life with a truly amazing woman."

"She’s motivated, hard working, funny as hell, and as dedicated to her sport as anyone I’ve ever known. Love you babe."

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