Install CoPilot Live USA, it’ll cost you about £20, and you won’t need to worry about any kind of data connections, or network coverage.
[edit] just had a look, and CoPilot Live Premium HD USA is £8.99, and if you want it with Canada it’s £13.99.
At that price it’s almost rude not to have it installed, then you have no worries about any data issues that might crop up.
Not to say that having a SIM with lots of data and calls available wouldn’t be a good idea, mind, but data coverage in the UK can be a nightmare if you need map data, imagine what the US can be like.