[General Questions and Answers] Planterz:
This is not what you want or need. This is merely a plan/minutes refill to an existing pre-paid account.

First you need a pre-paid SIM. Often called a "starter kit". You'll have to pick one up somewhere, like a Wal-Mart or a cell-phone store. I'd be very surprised if you couldn't find them for sale within 100 feet of baggage claim. And then you'd have to chose a plan. Most pre-paid plans that have 4G data allotments already include unlimited talk/text. Not necessary, as you say, but it might be helpful if you and your girlfriend get separated.

Whatever service you go with, the actual network will be either T-Mobile or AT&T. Your phone will be a GSM phone (I'm assuming), and AT&T and T-Mo are GSM networks. So assuming your phone isn't locked to a carrier within Germany/EU, it'll work just fine on AT&T or T-Mo or their MVNOs and you don't have to buy a new phone.

I would definitely recommend using an AT&T MVNO. The American West has huge tracks of land, deserts, mountains, etc. AT&T will have plenty of dead zones, but it'll be way better than T-Mobile. For what it's worth, you can get data in T-Mobile's dead-zones because they partner with AT&T with them for coverage. But it'll be "roaming" (T-Mobile doesn't charge for roaming), and at 2G speeds. I honestly don't know if this carriers over to T-Mobile's MVNOs though.

With that in mind, there are a number of AT&T MVNOs (as well as AT&T's GoPhone). I would recommend Cricket Wireless. They have outlets everywhere. Right now you can get set up with pre-paid SIM for $.99, and then choose the $50/mo plan which gives you 4gb of high-speed data, after which you get throttled to 2G speeds. If you need more high-speed, you can purchase another GB for $10. Don't let them charge you an activation fee - you can do it yourself (unless you can't).

Unfortunately, any plan with data

will be

monthly and will be purchased along with talk/text. Perhaps it's possible in the EU, but you can't simply purchase 5gb data that lasts until you use it up regardless of how long it takes. Unless there's something I'm unaware of. So that means in a 5 week trip you're pretty much stuck paying for 2 months unless you go a few days without service.

Regarding data and GPS navigation though, remember that navigation requires either the map to be downloaded beforehand, or a data connection. Using Google Maps on your phone to find a restaurant, site-of-interest, or hotel within a city is one thing. Using GPS to drive from Los Angeles to the Grand Canyon is another thing. Get a rental car with GPS and make sure the maps are all pre-loaded. You won't be able to rely on a phone's GPS for this unless you get an app that allows you to purchase the maps for the states you'll be traveling through and download them to your phone before you embark (find a public hotspot for this). Google Maps allows you to download maps for off-line use, but you're rather limited in the size. For example, I can fit all of Tucson in one map, but that's it. A city like LA or Phoenix wouldn't fit. Never mind all the miles inbetween.

Any questions, I'm happy to answer as best I can. Hope you enjoy your stay. I've lived in both Arizona and Colorado for many years, and I'm sure you'll be impressed by the west half of the US.