"There are only so many times you can watch a BMW go round the track — what more can you say? It's got four wheels and a seat. We all agreed that we've been doing it a long time and everything eventually runs its course. Besides, I'm 58 and I'm too fat to be climbing on to the stage."
It's not the last we'll see of the famous trio though; they've signed a new two-year deal with Amazon for at least four big-budget specials a year, which will be filmed around the world.
"They will be big exciting shows — proper event TV. We're not doing the big studios shows anymore. But we all love travelling and still enjoy each other's company after all these years.
"You get to the point where it's the specials, when we're out on the road in far-flung places, that we like doing the most.
"We did five in season three across the world in places like Mongolia and the US where we didn't have the studio tent. Amazon like them and they've done research that shows the audience like them, too."
Asked if they would have the same budget as the Grand Tour - up to $7 million an episode - he said: "Put it this way, Marvel will be jealous of our budgets."