I see on Strava that the segment KOM is 51.4 mph.

I cycled down it in 1992 on my Cannondale rigid mountain bike with 52t front ring and Specialized Turbo S tyres pumped to 90psi. There was a following wind as I started the descent and I was already spinning out on an 11t rear cog* within 20 metres and just tucked and plummetted! It was about 6am on an April morning (Saturday) so no cars about. I didn’t brake at all on the main curves as I was scared my cantis would get hooked under my rims (MA40 MB). I used the whole road to straighten the bends out. I HAD to brake as I approached the AA box as the corner was coming up frighteningly fast and I was STILL spun out!

Thing is, my magnetic speed computer told me 58mph* (I recorded it in my diary!) and that is the speed I have been boasting about all these years!

So, how fast DID I go?

Someone will be along to tell me what speed that gear combo is capable of attaining at maximum leg spinout.

*Addendum:
Did Shimano produce an 11t Hyperglide top cog in 1992? Might have been 12t although the fact that I had a 52t chainring will tell you how crazy I was about getting massive gears! I had an 11t as soon as it was available.
Yes, I was VERY careful to set my tyre circumference accurately.

Slightly OT, but me and an old mate cycled down the hill in Dartmouth going past naval college.

52 tooth again; Cannondale M500 with lots of purple.

We both got 52mph readout and overtook a coach!

It bloody was at 16 I’ll have you know! #Shelteredlife

try this, if you know the gradient of the hill, (its often on a road sign on steep hills) this will tell you what should be possible.
i compared it to the reading off my magnetic cateye from c1996, and after inputting data about tailwind (considerable, in my case) it was close.
http://www.kreuzotter.de/english/espeed.htm

Veloviewer puts the steepest section at 19.4%

I dunno if I’m doing it right but I got 62.2mph…

EDIT: that gear combo at 120rpm gives 44 mph. Spin out at 140+ ? Hmmm…

The 51.4mph being an average, it seems pretty plausible you did a maximum of 58mph, but that doesn’t mean you’re 15% faster than the KOM.