Garmin Nuvi Hardwire Cable (Mini-USB connector) for Garmin Nuvi
The power through the 755 or 855 Nuvi series puts the unit in data mode thinking it's connected to a PC rendering the Nuvi inoperable.  I did get it to work once on the Nuvi 855, but the Garmin folks (thumbs down for Garmin engineering) made the 855 so that power through the USB port decreases the screen brightness so the display can't be seen in daylight.    Do NOT get this for the 7xx or 8xx series.