My Zumo 660 is loaded with map CN Europe NT 2017.20 3D on the main device plus City Navigator Europe NT 2017.2 on the 8GB SD card.
Garmin Express tells me that the 660 has the latest maps and will not offer an update even though my PC and other devices carry + use City Navigator Europe NTU 2018.1.
Any suggestions as to why a Zumo 660 is being "left behind" in the version race?
Yes - here's the reason why:
The 'NT' and 'NTU' cartographic updates are two different products, and they have their own update cycles.
NTU (the 'U' stands for 'unicode', which supports character sets other than the Roman alphabet, such as Russian, Greek, etc.) is updated 4 times a year, and NT (which only supports the Roman character set used in English, French, Spanish, etc.) is updated twice a year. The NT updates are identical in their content - roads, points of interest, speed limits, stuff like that - to the NTU updates that are released at approximately the same time.
The 660 cannot use the NTU updates, because the 660 doesn't support the use of unicode characters. Hence the 660 only gets two updates per year, those being the NT updates.
If you have CN EU NTU cartography on your computer, it's because you also have a newer Garmin device that uses the NTU maps. You get 4 updates a year on that device. You'll only get 2 updates a year for your 660, because it can only use NT, which is only released twice a year.
Hope this explains it well enough.
Michael