I cannot imagine how anybody can stand the constantly changing value of a digital speedometer as it flickers around whatever your intended speed is. Give me a needle any day.
I have a digital speedometer with a 7 segment display in my Citroen and it doesn't flicker at all. It updates roughly once a second or so -- quick enough to feel responsive, but slow enough to not annoy me.
My 2 cents: generally the speedometer is always somehow just enough out of "default driving view" that I don't really see it, I have to actively look at it. And when I do, a plain number is easier and faster to parse.
