User MCC kindly provided these instructions...

I'll explain the procedure I'm using and hopefully it will work for you.

1. With the Nuvi 255 locked onto its satellites press the car icon to get to the Location page (My Location, Nearest Address, Nearest Junction down the left hand side of the screen, buttons for Hospitals, Police Stations and Fuel down the right side.

2. This is the sneaky bit. Press the screen ONCE just beneath the words "Nearest Junction", then press the screen TWICE just beneath the words "My Location", then press the screen ONCE again just beneath the words "Nearest Junction". This won't work in indoor mode without a satellite lock.

3. You will now get the "secret" Logging Control page. Press the "Start Recording" button and it will start logging your trace. Press Stop when you have finished.

4. To retrieve your trace plug the Nuvi into your computer's USB port and use Windows Explorer to open the "debug" folder. Each trace will be saved as a "gps.bin" file in its own subfolder, each uniquely named like "20091023_061223".

5. To convert the Nuvi's gps.bin file to a generic gpx format file I'm using a programme called wNutrak which I found on the internet. Works a treat.

I've noticed that while my Nuvi is logging a trace the car icon on the screen tries to lock onto existing roads but the trace data itself is real. The logged points are also close enough together for good accurate mapping. Occasionally I do a three point turn on a road or track or nip a short way down an unmapped side road or track and these little manoeuvres appear perfectly on the traces when opened and viewed on Google Earth or uploaded to OSM.