I have a 1970 Speedmaster, which has recently had a full service. During the past week, I have been keeping track of its timekeeping accuracy using the iPhone Watch Tracker app. I'm very impressed. A 45 year old watch is managing to average a loss of less than 2 seconds per day. The watch was left dial up every night, wound in the morning, and work all day.
That's a cool app.. I'll have to get it. I love mechanical watches.. amazing what goes into them and how well they work.
I serviced one of these for a Rolex collector, several years ago. He tells me it is as good a timekeeper as his best Rolex Chronometer, and better than several others. Mine is now 26 years old, and one of these days, I am going to get around to actually checking its accuracy. It is seldom on my wrist for more than a few days at a time, but I should plan on a long term test. I do know, however, that it is very accurate.