The last computer I used on a mountain bike had indicated all kinds of wonderful top speeds, even when I was nowhere near them. Don't use computers for quite a while now. Of believable readings I seem to remember on a steep dirt road (fortunately straight and flat) getting somewhere around 44, at least it seemed scary enough to believe. That was flying. Can't even imagine how those guys racing the original Kamikaze race on Mammoth used to do it (hitting 65 on rigid bikes, probably without discs...sound fun?).