It used to be that you could buy a North America map designed for another unit such as a 720, 920etc, and then just download it to a bigger SD card, and it would work fine on the TT1. Unfortunately, that is no longer the case. Last year, TomTom decided to make maps device specific, so you have to buy a map specifically designed for your TT1. Unfortunately they don't make NA maps for the TT1. You could try buying a more up to date US/Canada map, such as version 710 or 715, but it still won't be as comprehensive as a NA map.
