Reading this thread on the airplane on my way back from brrr... cold Cleveland last night was a nice warmer.

57mph is very fast but doable. The closer I have been to that was a bit less than 55 on the back side of Monitor Pass at the DR. Monitor offers pretty much ideal conditions and it felt fast and different from any speed I had ridden before. The fastest I have been on local rides is about 48mph going east on Rheem in Moraga, it's very steep at the top, straight as an arrow, on nice pavement but short. I span like crazy at the top, tucked low, and let it go. It felt fast but didn't offer enough length to really get to the level of Monitor. It takes time to gather enough momentum to break the 50 barrier.

Anyway as much as I would love to say 'yes you did it' I am skeptical and attribute the error to the Garmin Connect software you are using, not the Garmin unit. I exported the file as a TCX file from the link you provided and imported it in Ascent which I think is pretty accurate. All the numbers match except for speed. Garmin Connect shows 9.6 mph average and 57.1 mph max while Ascent shows an average of 11.8 mph and a max of 45.91 mph.

Ascent has an option to calculate distance and speed from the rear wheel sensor data. I tried both options on your data, the results were very similar with only 1 mph difference. I always turn the sensor data option on as a GPS sometimes looses contact under heavy tree covers.