I contacted Garmin and asked about using my Nuvi on six volts, they said that the cig lighter adapter plug that comes with the unit reduces the voltage to five volts at four amps and should work OK, at idle the voltage could drop and the screen will go to the Garmin and would show it connected to a computer as the charging rate from a computer is very low. So god to go with six volts and the plug adapter.
Rick
As I have written elsewhere - this complies with my experience.
Go try it. It can't hurt anything.
That should read 1 amp.
Rick