my 66 managed 46 kph a couple weeks ago,after 1/2 hour ride (still in break in mode)
i had to crack it open on a little downslope past my house,i was cruisin.
this is on gravel,btw.i dont get much pavement driving.
I've built my own Zenoah 30cc lh/rc hybrid with a walbro 257 HP carb/ ada filter and an old crusty ADA s2 pipe and i cruise at 25, 35 is no problem and 40 is possible if i pedal really fast. I run a 14t pinion sprocket and a velocity built HD wheel from gebe.
I like to cruise at 25/26 on either of my bikes, my op with the gt2b grubee 48cc and 44t rear sprocket does 32on flat ground. My crandbrook with the bgf48cc and 41t sprocket does 31, I have ported both and trimmed the piston skirt on the bgf.