there's a bunch of threads about how fast people have gone. i'm not doubting you outright, because i've gone fast, too, but as the saying goes, without proof, it didn't happen.
the rpm's needed to spin the chinee motor up to 50 is almost impossible, without extensive (and expensive) mods.
everyone's waiting for someone to show undeniable proof of 50 (or 40, for that matter) and so far, there's been none.
plus, the general consensus here, is straightway speed, level ground. down hill doesn't count.
my brother had a Hutch HPV "superbike" (google it) and we could hit 50 on a straightaway with no motor, and that's fast. scary fast. i mean, eye-watering, bone-jarring, death-fast on a bicycle. nothing on a regular bicycle is designed to go that fast, and it shows when you start hitting 30+ on a beach cruiser.
and i'll ask first, because everyone else will sonner or later, "what kind of bike? what kind of mods, how'd you calibrate your speedo...etc..."
as i said, i'm not calling b.s. on you outright, but others will, and i'm just giving you a heads up...