Flenser, the problem with most of the glass made for Poser props is that it is only single sided.  That will throw off the refraction in a huge way.  If the windows on that car only have one layer you would be better off setting the material to Lux's architectural glass, or setting the IOR to 1 so it won't refract.

That's going to be something to keep in mind for most of the glass items we use in Poser.