William Bruce wrote:
I've just bought a TomTom GPS for my car with preloaded road maps of
the US and Canada. My question: Will it work in my 172 at 3,000 to
5,000 feet, showing the roads below, etc?

The only problem you will have is the issue with the TomTom telling you you
are on the road. It is designed to assume you are on the street or highway
and if it calculates something different it puts you on the closest road.