Green bars or satellites page not always are reliable to measure GPS accuracy as said above by Angelo, if you want for real to check Nuvis' accuracy, enable tracklog, put them "off road" or pedestrian mode and drive a route. After that compare tracks using Mapsource and Google Earth.
We learnt we can have some surprises.
You can see some examples in other thread regarding sats accuracy in converted nuvi 37xx.
Clearing non vol after every firmware flashing is important for good nuvi's operation too.
Some tinted windshields can influence sats reception, normal glasses dont.