How come most of the used boats I look at the speedo is faded dark? but every other gauge looks fine? i would think they all get the same sun light and are made by the same people.

I have noticed that too. Not sure is why though

If they are the liquid filled, then that is why. The glycerin in them seems to make them change color/fade faster.

WHy is it liquid filled? i know the speedo works off water pressure... but i would thing the needel moving in water would just be more restrictive.

The glyceren is there to steady the needle, and give you a more stable/accurate reading. It just makes everything smoother and more vibration resistant.