I feel confident I have not exceeded 55 kph but I don't have a speedo on any bike. If it feels fast, it is fast, as far as I'm concerned.
But I did see a reading of 64 kph coming down Ventoux last month. (Not hugely fast I accept although I could have gone faster if I was not on a rental with an uncomfortable saddle with front brake controlled by left hand!) Bizarrely, I kept looking over my shoulder to see if anyone/anything was coming up my backside. Sure enough, another cyclist soon overtook me. I then understood why commentators refer to some pros (e.g. Cancellara, Nibali) as 'good descenders'. Not a description I'm likely to acquire.
And then I felt a very sharp stab in the chest. I looked down to see a wasp trying to escape from inside my jacket, having decided to test its stinger. Not great timing with a sharp bend approaching!