I'm guessing that moving red line on the speedo is the "drive faster than this and you're wasting gas". If the answer's in the OM, I haven't found it. Which doesn't mean much (see post re: clutch pedal).
The line is a red line pointing radially towards the center of the gauge. It's not a repeat of the speed limit info. It moves moves around, so it's not a form of the setting for cruise control. BMW has what, in the airplane world, is called a bug that moves to mark some setting. In BMW's case, the cruise control setting. I haven't finished playing around to see what moves it. I decided to take the easy way out and ask here. I tried the OM in the car and... nada.