You're sure about this? The 660 may not display the achieved top speed and display it on a summary page - but if you look at a recorded track in Basecamp or a similar software, there should be the archived speed for any track point recorded!
@mad-dawg - i still can't see, were the problem is? The Zumo XT has a data page, were you can see the achieved top speed of all your rides since you bought it (or have done a reset). Nobody, which accessed this page can see, when or where this top speed has been driven! You could be on a race track or on a private road or on a german highway time ago, where it is legal to drive at such a speed - nobody can tell...