Sony's flagship PlayStation racing franchise Gran Turismo is approaching 68 million lifetime sales.According to data revealed by series creator Polyphony Digital, 67,890,000 Gran Turismo games were sold between the original's PSone debut in 1997 and September 2012.In comparison, Sony said in June that it had shipped over 21 million God of War games to retailers worldwide since the series debuted in 2005.The original Gran Turismo has sold 10.85 million copies. 2001's Gran Turismo 3 A-spec, the first game in the series to be released for PS2, is the best-selling GT title, having shifted almost 15 million copies. 2010 PS3 release Gran Turismo 5 has cleared nine million sales. Here's a rundown by title:Gran Turismo: 10,850,000Gran Turismo 2: 9,370,000Gran Turismo 3 A-spec: 14,890,000Gran Turismo Concept Series: 1,560,000Gran Turismo 4 Prologue: 1,400,000Gran Turismo 4: 11,600,000Gran Turismo 5 Prologue: 5,350,000Gran Turismo PSP: 3,860,000Gran Turismo 5: 9,010,000Total: 67,890,000Gran Turismo series producer Kazunori Yamauchi confirmed that Gran Turismo 6 was in development back in 2010, although he has left fans to speculate whether the game will arrive on PS3 or PlayStation 4.This August Yamauchi suggested that the Gran Turismo 6 release date will remain a secret until Polyphony can be certain of its launch plans, telling press at the GT5 Asia Championship 2012: "If I carelessly say when it will be released, you'll end up with people saying things like 'it's been delayed again' so I will no longer say at this type of event.