This is why I don't use the speed display, except under very specific circumstances (late or early in the day, when I'm wearing sunglasses, and the analog display is too dim to see against all the ambient light).
Watching the display spazz between 44 no 45 no 49 no 47 no 44 no 47 no 49! drives me crazy. I feel compelled to hold precisely 45 and my inability to do so makes me feel inadequate.
I like the analog display because it smooths out the speed. I find the car (in sport mode, at least) is a bit of a challenge to keep at a constant speed, and in addition the car likes to go fast and I find myself creeping up (it was the car's fault, officer!). The analog provides a coarse visual clue of my speed that feel is much easier to track.
But anyhow. My digital display seems to match the analog pretty closely. I'll stare at it today on the commute. If there is a big discrepancy, I don't know what can be done about it. My advice would be to use one or the other and retain your sanity.