What is the source to the fact that Tiberius won the quadriga race? as far as I know Nero was the only Julio-Claudian emperor who won an olympic competition.

"He also drove a chariot in many places, at Olympia even a ten-horse team, although in one of his own poems he had criticised Mithridates for just that thing. But after he had been thrown from the car and put back in it, he was unable to hold out and gave up before the end of the course; but he received the crown just the same." (Suetonius, Life of Nero, 24.2)