About 10 years ago 72mph in a downhill sweeper on a road machine. In theory could have gone faster but the wheels were barely in touch with the ground so I backed off.
This year to date 65kph on a mountain bike with oversized road tires. Now the issue is gearing, not stability and I'll be installing a larger chainring. The suspension and tires make the bike dead stable and 100kph+ should be a snap.