There are slight variances from brand to brand, but on the whole most new vehicles leave the factory with around 75% light transmission.
So remember, when you add film to your car, the film darkness combined with the tint already in the glass is the final number of visible light transmission (VLT).
This front door on a Kia Soul is an example. The film was a 38% ceramic. The owner wanted to keep the front doors within the law for Indiana, which is 30% VLT. The rear windows are bone stock, and meter at 25%. Now they are only 7% darker than the fronts, and the whole vehicle is completely legal.
Bloomington Window Tint – Kia Soul with Ceramic Film 