Check the accuracy in the sat page [hold the bars on top LH corner on the main screen to access it]. Many ppl complain the they had 3m accuracy routinely before conversion but not afterwards. See this thread: [Only registered and activated users can see links. ]. It seems to depend on the gps chip fitted, one is just more compatible with 34x0 software than the other apparently.

The GPS's speed reading is certainly quite accurate provided the sat fix is good. Speedos in cars seem to read up to 10% over in many countries. It varies though, my late model Subaru Liberty reads about 8% over, yet my 1992 L series wagon reads smack-on. Go figure. Cars i've hired in the States all seem to read accurately, same as a GPS anyway. Id trust the GPS reading before the car speedo once it's got a stable fix.