Way back in the 1980's, I had one of those speedo's that ran off a little wheel positioned against the bike front wheel. At speeds of over 30 mph the little wheel used to skip and wouldn't give a stable reading. The solution was to take one hand off the handlebars and reach down to hold it in place whilst pedaling like a demon on fast downhills...there was a stop for the needle just after the 40 mph mark and I regularly hit that. Stupid doesn't really start to cover it.
Nowadays (using GPS), 35.1 mph on the hybrid, which felt like it was on the verge of running out of gearing almost as quickly as I was running out of road. Disk brakes = good.
And 39.1 mph on the road bike through the dip at Wothersome, near Bramham Park. Focused my concentration a bit there as the road was a little bit damp in the bottom of the dip and the gentle curve onto the uphill section seemed quite a bit tighter at that speed...
I'll have to have another go next spring when it's dryer. It's a 60 mph limit so I've got a bit left to play with.