hmm, I'd disagree mildly. The altitude readings are (at least on my GPSs, and I have two, never mind the built-in PoS in the N95) sort-of accurate. Having said that, the GPRMC message provides information in knots, which has to be converted. BUT: a 30 camera in a 30 zone is IMO a) not playing fair b) not reasonable and there's no way on this planet that will be accurate.
your GPS can only ever be as accurate as the signals it gets and the programming in it. To assume it's more than +- 2% accurate is wishful thinking. It might be, but then there's calibration, heat, tree cover, number of satellites.... so it may well vary from day to day, never mind hour to hour. That's also why the Audi nav systems incorporate a reading of how large the wheels are, as they also take that into account.
OTOH, I know my N95 / LD-3W system got confused last night, believing I was the other side of a crossroads whilst sat at a traffic light, so... I'll continue to plan my trips around the cameras, using as many backroads as possible. They're more fun anyway.
Lack of limit is one of the reasons I find German Autobahns much easier to drive, but even then I find myself doing 140-150-160, depending on the car. They are also designed to keep you awake.
Sweden is soooo boring, with the 110, but that's why I have cruise. Same here, but the backroads are far more rewarding. On the main roads, it's always a challenge to stay awake, on the back routes, it's about pushing and trying to remember not to take the pi$$ with the speed. Easy when you have to brake every 30s
Bret