Franchitti is a major driving star in the United States

The Scot won the IndyCar series title and the Indianapolis 500 race this season for Andretti Green Racing.

However, next term the Edinburgh driver will race the number 40 car, in the Nascar Sprint Cup Series.

"It's going to be the toughest challenge of my career. I have no experience and I will rely a lot on my new team-mates," Franchitti said.

"It will be difficult but that's what I'm looking for. Not defending the Indy500 title was one of the hardest parts of making this decision.

606: DEBATE Give your reaction to Franchitti's move

"As far as leaving IndyCar, I think the series is much bigger than any one driver and they have a lot of great drivers coming up over there to take my place."

Franchitti has signed a multi-year deal beginning in 2008 and is the first European driver in the 59-year history of Nascar to join the series full-time.

The Scot's contract with AGR had not been due to finish until the end of the season but they agreed to an early release.