According to Official Guinness Records,
The fastest speed achieved on a monowheel motorcycle is 98.464 km/h (61.18 mph) by the UK Monowheel Team and Kevin Scott (all UK), riding WarHorse at Elvington Airfield, North Yorkshire, UK, on 20 September 2015. A major challenge to monowheel riders is avoiding a "gerbil incident" – that is, accidentally looping around with the outer wheel instead of remaining seated at the base of the vehicle. Kevin achieved his record-breaking speed without impersonating a gerbil. Also in the record-breaking team were Peter Orton, Peter Kay and Tim Mann. Full story: Video - Fastest monowheel motorcycle speeds into Guinness World Records 2017 book
For a complete list of 2015 records, please visit 2015 Guinness Records in United Kingdom.