running in

dated The process of driving a car with a new engine at low speeds for a period of time, as to acclimate it. Primarily heard in UK. You kids today probably don't even know what running in is! Getting a new car was a very different experience back in the day.

running in, please pass

dated A sign posted in a car's window to indicate that it has to move slowly for a period of time because it has a new engine that must be acclimated. Primarily heard in UK. A: "I'm glad no one's honking at us for driving so slow!" B: "Oh, I put the 'running in, please pass' sign up!"