The Californian showed flashes of promise but struggled to get any consistent pace out of the car and left after the rain-soddened 2007 European Grand Prix, when Red Bull decided to promote Vettel to the junior Toro Rosso team.
He is more philosophical when reflecting on his own F1 career and says there are plenty of brilliant drivers who have never won a title, adding that timing and circumstances have to be correct to win.
Speed added: "Achievement in Formula 1 is so difficult because there's so many guys that are in Formula 1 that don't have a Formula 1 World Championship, but they're amazing.
"So depending on your era and your timing of when you're in there or when you're not, it’s incredibly important."
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