Standard for comparison of antenna behavior outside and inside a vehicle

HI!
I need to compare the behaviour of the same antenna while it is located inside
and outside a vehicle.
I have to determine the difference between the 2 position (the environment
in which the vehicle is moving is urban).

Is there any standard to look at?
Thanks

Simularors like Feko..IE3D..etc will be useful for you. draw vehicle. place the antenna inside. simulate radiation pattern.