In the Polaris Challenge in the Keilder Forest in 1992, I recorded 50mph on my speedo off road. It was a very steep and quite twisty fire road off of Hindhope Law (425 metres) that was really icy. I was scared to use the brakes and pedalling seemed to give me more grip than trying to just coast.
When I managed to stop at the bottom, I checked the max speed and thought it must be wrong until the next guy down came screeching to a halt and checking his speedo yelled "Wow! 48 mph", so I guess it was true. It was on my Cannondale (in those days fully rigid) with a 48/12 top gear.
Ironically the fastest I have knowingly been on tarmac was only 48 mph, again on the Polaris, this time in 2004 in the Northern Pennines. It was on the descent off Killhope Summit (627 metres) on the A689 heading east towards the village of Nenthead. This time I was deliberately going for a record and only failed beating my previous best because my chosen route meant I had to turn off on a side road half way down. The road runs straight down the hill so If I had let it run all the way into Nenthead (at the bottom) I'm sure I could have got nearer to sixty. This was on my Scott and was on a 48/11 (retro chain rings (48/38/28 ) but a modern 8 speed cassette).