Maybe something wrong with this Nuvi's Sqlite application and associated files.

Speed Corrections are housed in /.System/SQLite/SpeedCorrections/speed.db

Maybe this file is corrupt.
Try to delete this file.
Also check directory structure.

Check if you have 2 maps of the same area loaded in the Nuvi.

Some roads do not let you change the speed, The road where I live shows + all the time no matter what I do, Other roads I can change.