Dear James Tan,
All map image is usable for your Nuvi 40/50..the different is the JCV file,you can spot a different between JCV file size..is seperate / different in size because of the Junction View ( JV ) size,you still can use the smallest one file included JCV file into your Nuvi40/50..just setting the JV appearance to the half screen,so no issue of pixel lack while you use the smallest one file.I'm also didnt ever use JV functions in full screen size..but its depend to you,my opinion is base on my everyday usage.TQVM