OK, are we talking 47 MILES per hour or KILOMETERS?
Unless you are running a really small back sprocket in the back direct drive and pushing 10K RPM on tricked 66cc a 26" bike wheel that speed claim is doubtful to me.
Tucked in behind a big truck and such maybe, but about 35MPH is when air resistance kicks in hard.
I've built at least a dozen shifters with a 66 that pass 45MPH in short order but that is because it has gears and even with gears you got to get low and tucked to go that fast.
Personally 35-40 is just dandy to me so long as the bike has really good wheels and brakes which, well, all my builds get.