State laws differ however most require that a car have a temporary or permanent plate on a vehicle in order to drive it.
For example Wisconsin Department of Transportation states that WI law requires buyers of cars and small trucks (8000 pounds or less) to display a permanent or temporary plate within two business days of purchase. Driving without plates is subject to a maximum fine of $200 in Wisconsin.
To find out what your state's requirements are regarding showing a license plate or tags on a vehicle and what the penalties may be for driving a car without plates on it, check with your state's Department of Motor Vehicles.