according to internet, 63miles an hr .That's not bad for a dirt bike in 68.
250 class
Taken from Bonneville records (that still stand today)
S-AG 172.455 Mph Vesco, D. Yamaha 1972
S-AF 189.529 Mph Vesco, D. Yamaha 1973
Code...
S = Streamliner
AG = modified+ normal petrol
AF = modified + other fuels