it's possible, but...
what other mods to the engine have you done?
what kind of bike?
there's a lot of factors. a well built bike with a lightweight rider can get into the 40's (or faster) but if all you've done is change the sprocket, it's unlikely.
my first bike was a stock 26" cruiser with a 36 and i got 38mph.
my race bike is a heavily modified china engine on 20" wheels with a 36 and it tops out at 43.
my old 26" with an insanely modified motor and a 28t blew up at 62.
aleman witnessed it at 54.
i've seen and had bikes easily go over 40, but they've all had a lot of work done to them.
all that being said, you mighta got what i call a "lucky motor." sometimes you'll get one that goes fast right outta the box.