Compressing Music Files For Thumb Drive In Car

I just bought a new Jeep Grand Cherokee and the sound system is excellent.......for a car. I have a ton of music in JRiver, and most of them are large files. I'd like to compress them for my 128 gb flash drive. I think I could fit more music on it, and the lowered sound quality for be fine....because it's a car stereo, right?

Can I do this? If so, how?

Thank you all in advance!