I am going to Italy and taking my Garmin nuvi. The specs say it operates on 12/24vdc. But what is car voltage in Europe? I seem to remember something about 6v batteries from the foggy distant past.
I doubt that 6v is used in any modern car anywhere in the world.
Twelve volts is pretty much universal for passenger cars and light trucks. There might be some exceptions, but I don’t know of any.
Six volt systems were not uncommon in the 1950’s and earlier, both in the U.S. and abroad. To my knowledge, the last holdout was Volkswagen, which used 6 volts through 1967.
Although you might find 6V still being used on old cars (e.g. certain MGBs which had two 6V batteries in series either side of the prop shaft) you will not find it on any current production model.
They did indeed use 6V batteries, which were becoming a rarity at the time, but it was a 12V system (6V + 6V in series = 12V).
My US bought Garmin nuvi works fine in France.
Does your Nuvi have the roads for Europe?
You have to buy the map separately.
Some old British cars were “positive earth” which means that the positive terminal of the battery was connected to the car chassis instead of the more usual negative earth (ground as Americans would say). This is going way back when I worked at a shop installing car radios when I was at high school back in New Zealand. Most modern radios etc were -ve earth only. I think the +ve cars were old enough to have a generator rather than an alternator. It was easy to change their polarity. You reversed the battery and then you had to zap some current through the generator with jumper leads which made a big spark and reversed the magnetic field in the generator.
What do you know, there’s actually a Positive Earth Drivers Club 
I owned a Ford Cortina that was positive ground. It was also evidence as to why British people drink their beer warm. The electrics were execrable. Any culture capable of producing them would be unable to produce refrigeration.
Lucas, the Prince of Darkness and Warm Refrigerators
Well, my Garmin nuvi worked just fine in the rental Citroën. Vacation pix to follow soon.