What will 128 kbps sound like in a good car stereo?

I have a huge iTunes library and I'm trying to fit all of my music onto my iPhone. I have a very nice car and an amazing stereo in it, and it sounds wonderful right now at 192 kbps. I'm satisfied with the sound at the moment. If I compress the music to 128 kbps, will there be an audible sound difference?

The difference in quality can be noticeable. Your equipment can help in some cases. Some head units/processors can "fill in the gaps" on music files with low bit rates. Personally i don't care what any head unit claims. I always use 320kbps. I have an expensive set up so i figure why go cheap on the tracks i listen to? I can clearly tell the difference between high quality and low quality bit rates. If you got a kick @$$ stereo, let it shine with high quality music.