what is the latest US map version that is smaller than 1.6GB?

Put the map on a MicroSD card instead. 4GB cards are dirt cheap, and even 8GB are affordable. With a MicroSD card inserted, Garmin's map update software should have allowed you to install the entire map.

As to returning to the old maps, once you do the update from Garmin you can't revert unless you have a backup.

Adding to what Strephon_Alkhalikoi said. Whole idea of map update is to get mistakes in routing removed and have new roads added. It may seem not important for you at the moment but it can be real problem. As long as you are driving in area that you more or less know, mistakes in routing are not big problem. But once you get into area that is unknown to you old map can give you real run for miles just because it may not have newest roads coded.

If you are driving on limited area you can consider installing only part of map. Like a few states. If you want everything: put it on external card.

ff....

Welcome to The Factory.

We may be of more use to you if you tell us which Garmin device you have. If yours is one mentioned in footnote 2 of this link:

https://support.garmin.com/support/searchSupport/case.faces?...

then it's super easy to auto-update to the latest map directly to SD card. It's also possible to update maps to SD for other nuvis with a bit more effort.

Also, you may want to remove (after a computer backup of your GPS) unneeded files like foreign language voice, help and text files which will give your device extra storage which may be enough to update a map internally.

As has already been said, 4 or 8GB SD cards are very cheap giving you a great way to have the latest full map update on your system even a GPS with limited internal storage. Also as already stated, you can not go back to a previous version unless you've made a computer backup of the old map which you can restore to your device.

But...to almost answer your question, the .img file for 2012.40 was 1.66GB (and for 2011.40 was 1.26GB) so for the .img map file size being under 1.6GB, my guess will be 2012.30. This doesn't include other files that come with some nuvi models like the Junction View file, but not knowing your model number, I don't know if your GPS offers JVs.

I highly recommend that you do what the others have suggested but another option would be to choose and download the region you'll primarily be using.

Cheers

https://support.garmin.com/support/searchSupport/case.faces?caseId={80e6d330-f2e0-11dd-f45b-000000000000}