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."

49-year-old Stewart had just celebrated his 1-year dating anniversary with 32-year-old Pruett ... when he decided to put a ring on it this past weekend.

Pruett was so stoked, she casually showed off the hardware on her IG page ... and yeah, we noticed!

The couple met in 2019 -- but didn't go "official" until 2020 and reportedly spent the majority of the pandemic quarantined together ... and apparently, it's been going great!

During their 1-year anniversary last week, Tony posted a love note for his lady ... saying, "The last 365 days with this beautiful lady have been the highlight of my life."

"Cheers to one year together and many more to come!"

Looks like he meant what he said ... he's hoping they spend the rest of their lives together!

Stewart is a 3-time NASCAR Cup Series champ and he was inducted into the NASCAR Hall of Fame in 2020.

Pruett is also BEAST on the track -- the NHRA Top Fuel driver has 11 total wins in her career.

Stewart has been engaged before ... he proposed to actress/model Pennelope Jimenez back in 2017 but ultimately called things off in 2019.

As for Pruett, she's been married once before -- but that ended in 2019.

No word on when the race car driving lovebirds will formally tie the knot -- but we're guessing it'll be one helluva party.