I am having this exact same problem in my 2011 328i convertible. The cruise indicator usually shows the right speed when I first turn on the cruise, but when I hit resume, the indicator screws up about 50% of the time. Differences in the actual set speed and the indicator can vary anywhere from a few mph to >20.
The dealership said they "re-programmed" things the first time they looked at it...and it did the same thing driving away from the dealership!
Has anyone else had this problem? If so, how was it fixed? What was the root cause?
When it screws up, I've noticed that the indicator appears to slow down or get stuck when it first starts to move around the speedo to the indicated speed. Something loose on the indicator ring maybe?? :dunno: