EMP is largely line of sight from the point of the nuke. A single bomb detonated about 200 mi over Kansas has the potential to impact most of the US. A lot depends on the strength of the pulse, distance, design of the electrical device, connectivity of the electrical device, and even the landscape. Large concrete buildings, mountains, hills and so forth will provide some protection to things in their shadow. Surge protectors will not help.
To be effective a faraday cage has to be 100% coverage. So while the metal car body may provide some protection in theory the cars body is connected to the electronics in the car. The computers in cars are fairly full of sensitive components.
Unless they use multiple EMP bursts it would not be 100% and the damage would decrease as the distance increased. Of course the electrical grid is all connected so it would probably all be destroyed by the pulse or the resulting chaos.
>> the electrical grid is all connected so it would probably all be destroyed by the pulse or the resulting chaos <<
Agreed.
And IMO, no matter how many other devices survive unscathed, the collapse of the power grid — all by itself — would be enough to bring the USA to its knees.