@artful53

You can use [Only registered and activated users can see links. ] to find the details of your current basemap. It supports drag & drop to the Files tab and then choose details button.
The basemap is showed when zoomed out and detail maps just kicks in at closer zoom levels.

The smallest WorldWide (non routable) basemap I have is 10 MiB (10,493,952 bytes). Guess that you have a regional one for UK since the one you have is smaller in size.
Example with gMapTool details:
Code:
Please Login or Register to see the links
There's a collection of basemaps in the map index at [Only registered and activated users can see links. ].

I would choose the [Only registered and activated users can see links. ] for nüvi 660. If you want the latest and greatest WW DEM basemap instead you can try [Only registered and activated users can see links. ]. DEM (Digital Elevation Model) would show 3D simulation with shadowed topographic contours for supported models.