Car window clings are a buzzed-about product that exist more in theory than in actuality. Customers love the idea of a window sign they can place on their vehicle and then remove whenever they want-if they're advertising on their personal vehicle, maybe they want to take the sign off when they're not at work? This is certainly a valid request, but, as of right now, there's not really a great way to fulfill that request. If removability is your main concern, you'll want to choose a car magnet. This doesn't go on the window but it is easy to take on-and-off.
Static cling material technically can be applied to a vehicle window-it will "cling," at least until the vehicle is moved. Since static cling doesn't have an adhesive, there's nothing holding it to the vehicle while it is in-motion. In addition, strong rain, wind, or snow could cause the cling to come off of the vehicle. Another possibility is vandalism-anyone could walk up and peel the cling right off of the vehicle!
Window Cling Material
Customers often counter this problem by ordering a window cling to go inside the car. Once again, this will technically work, and it will solve the problem of weather damage or vandalism, however a new problem is created-tinting. Almost every vehicle window is tinted at least slightly, especially rear windows, which is where most car signs are placed. When a layer of tinted glass is between the window cling and the reader, visibility is very difficult, especially from a distance or if there is any sunlight on the window.
All this being said, there are a few situations where a car window cling would actually be useful. If you are not using this product for one of these reasons, you are strongly encouraged to pursue car window decals, which actually have and adhesive.
Oil change decals-In this case, a car window cling is exactly the right product. These small static clings are added to the top of your windshield (which is not tinted) after an oil change. They are removed and replaced every 3 months-a cling is much easier to remove than an adhesive decal.
Side window signs-If you want to decorate or advertise on the side windows of your vehicle, a cling could work as some side windows aren't tinted. You'll need to install it on the inside of your vehicle, though.
Parking passes-If you work for a company that issues parking passes (apartment complex, school, large corporation, etc.), a car window cling could work for you. These do not need to be seen from a distance, just by a security card walking or driving through the parking lot, so tinting isn't as much of an issue.
If you don't know if a car window cling is right for you or not, you can probably assume that it isn't. Go with something more permanent like a car window decal (unless removability is your main concern, in which case you need car magnets).