you can save the speed variable as a global variable:
import GameLogic as g
c = g.getCurrentController()
owner = c.getOwner()
g.speed = owner.speed
let this script run every time the speed variable schanges
then you let the speedo`s object check if your variable has changed:
import GameLogic as g
if hasattr(g , "speed"):
c = g.getCurrentController()
owner = c.getOwner()
owner.speed = g.speed
another way to do this is by sending a message to the speedos object with the property every time the property changes but i dont know if this works through different scene’s