The ETZ250 owners handbook gives the top speed as 125-130kph, depending on load, conditions and sitting position. Say 75-80mph.
A 1982 road test at MIRA gave 2 way average top speeds, full tank, of:
84.4mph prone
78.4mph upright
Speeds in gears at maximum power: 22,35,49,62,75
Standing 1/4 mile: 17.6 secs/72.3mph
Average mpg: 42.9
Speedo Ind/Act: 70/66.6 mph