If you like the idea of the free data-only SIM cards now available through Google’s Project Fi mobile virtual network operator (MVNO) program, just know that you won’t be getting 4G LTE service everywhere.

Today Google published a coverage map for the data-only SIM cards, and it’s revealing. If you compare it with the coverage map for regular Project Fi SIM cards, you’ll see that these new cards rely on 2G or 3G service for a whole lot of places around the U.S.

The 4G LTE service does show up in many cities and major thoroughfares, but once you go outside of those areas, things change.

In the New York and San Francisco metropolitan areas, the difference is not so big.

Google Project Fi coverage in New York with new data-only SIM cards.

Above: Google Project Fi coverage in New York with new data-only SIM cards.

Image Credit: Screenshot
Google Project Fi coverage in the New York metro area with its primary SIM cards.

Above: Google Project Fi coverage in the New York metro area with its primary Fi SIM cards.

Image Credit: Screenshot
Google Project Fi coverage in the San Francisco area with a data-only SIM card.

Above: Google Project Fi coverage in the San Francisco area with a data-only SIM card.

Image Credit: Screenshot
Google Project Fi coverage in the San Francisco area with a primary Fi SIM card.

Above: Google Project Fi coverage in the San Francisco area with a primary Fi SIM card.

Image Credit: Screenshot

But if you zoom out on the map, you can observe the pattern.

Google Project Fi coverage in North America with data-only SIM cards.

Above: Google Project Fi coverage in North America with data-only SIM cards.

Image Credit: Screenshot
Google Project Fi coverage in North America with a regular Fi SIM card.

Above: Google Project Fi coverage in North America with a regular Fi SIM card.

Image Credit: Screenshot

And in some metro areas the difference is significant. Take Phoenix for example.

Coverage in Phoenix for Google Project Fi's data-only SIM cards.

Above: Coverage in Phoenix for Google Project Fi’s data-only SIM cards.

Image Credit: Screenshot
Google Project Fi coverage in Phoenix with a regular Fi SIM card.

Above: Google Project Fi coverage in Phoenix with a regular Fi SIM card.

Image Credit: Screenshot

Same with Portland.

Google Project Fi's data only SIM card coverage in Portland.

Above: Google Project Fi’s data-only SIM card coverage in Portland.

Image Credit: Screenshot
Google Project Fi's regular coverage in the Portland area.

Above: Google Project Fi’s regular coverage in the Portland area.

Image Credit: Screenshot

So before you go online and order one of these new data-only SIM cards by navigating to the Your Plan section of your Fi account page, just be aware of what you might be in for.