On top of that, he has won the famed Daytona 24-hour race twice Car and also competed at Le Mans last year in Ganassi-prepared Ford, finishing third in class and setting the fastest race lap in the GT Le Mans class.
"I think Scott deserves more credit," Franchitti told NBC Sports in the United States.
"He's bloody impressive in what he's able to do in an IndyCar and what he's able to do in any car. He went to Le Mans last year and I believe he was the quickest guy in that particular car.
"He just goes about it with such a low-key approach. He's not a 'song and dance merchant' like (rival driver) Helio [Castroneves] for instance. You can't deny what he's done."
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Franchitti said Dixon's ability to extract the most of a sub-standard car was another mark of his world class status.
"Having been his teammate I know how bloody good he is and how hard he is to beat," Franchitti told NBC.
"I would say of all the teammates I've had, he's the one who can make the most out of an average car. Part of my job with working with the Ganassi team is helping him to push him to make the car perfect.....he's an exceptional, exceptional driver."
In the interview, Franchitti compared Dixon to Alonso, pointing out the Spaniard had also earned a reputation for enjoying remarkable success despite often having "off-the-pace" cars during his career.
"He's also got that thing with Alonso of never giving up. Every lap he's relentless and Scott's that way," Franchitti said.
"I've been on the losing side of that! He just keeps going."
Franchitti said Dixon was showing no signs of losing his competitive streak.
"Obviously he's been very successful and won a lot of championships and made a very good living, and has a very lovely family. But his enthusiasm and his passion for it and his commitment hasn't dulled at all.
"I spoke to him [a couple] days ago, had a quick chat. He'd dropped the girls off at school, and he's straight to the gym, every morning. Boom boom.
"He gets in that car and he's still hungry, he still wants to do the job, he still wants to be the best. It's pretty impressive."