Drive 50 LMT +nuvi 2557 LMT off by miles

I would try out both units at home outside (not in a vehicle) , in an area where they can 'see' the sky with no trees/ buildings etc. blocking satellite signals, if they get an accurate fix on where they are you are in a good place to start from, some car front windshields block satellite signals as do trees, buildings etc.

 

Installing new maps should not have this effect, I would suggest a factory reset, this will erase all user data so you need to backup all favorites, routes etc. before proceeding, with the unit off touch and hold the bottom right hand corner of the display, while still touching the screen turn the unit on, you should see a screen saying 'Are you sure you want to erase all user data' answer Yes