On the more open areas of Autobahn in Germany, it is not uncommon at all for that differential to be 50MPH or more. Those folks have been educated from the start to move to the right and let the faster traffic pass. They have very few accidents. Of, course it is very difficult and takes lots of training and serious testing to get a driver's license there. Here, on the other hand, you must be 16 and be able to see better than 20-200.
Grandma needs to stay in the right lane.