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!
Try WavePad Sound Editor, a free download.
db Poweramp is a great file conversion program, I usually use it to rip discs to flac but pretty sure you can rip flacs to MP3's and other files as well. Google it there's a ton of info or do a search here it's been discussed quite a bit.
+1 on DBpoweramp. I convert from wav and lossless flac to lossy flac to fit into smaller drives for non-quality demanding applications.
DPpoweramp is excellent, but I don't think you need to buy it. Do a search for free audio trancecoder for whatever OS you're using. You shouldn't have any trouble finding free alternatives.
You most likely will need either mp3 or mp4 files. Also, depending on your car, it may or may not be able to read and/or search files. Start with a couple of songs,then try a couple of folders and build from there. My final issue has been that many cars can't handle big thumb drives. Play with an 8gb first. Once you figure out your car's system then go bigger.
Free version of media monkey works well for bulk file format conversions but has learning curve. I used it to covert 1000s of files from wav to flac but does MP3 compressed also.

Dbpoweramp has cost but probably more user friendly. That's what I would use now.
I have a 2014 Grand Cherokee and the manual says that it will accept a 32 gb. card also it will only play mp3 files I created a folder of the mp4 songs that I wanted then converted them to mp3 works fine.
Get dbPowerAmp and compress your files to 320K Variable Bit Rate. This will give you *excellent* sound quality for on the road listening.

And if the 32 GB limit mentioned is true, simply partition your 128 GB flash drive into 4 32 GB "drives" as long as the Jeep's audio player will recognize the 4 virtual drives.

If it won't, then just buy a 32GB flash drive and load your files onto that. I bet a 32GB flash drive costs about $8.00 nowadays...

I'm using 32GB USB drive in my Honda Accord. Music is in 256K VBR AAC format and sounds great. The only problem is that Accord does not show directory structure. No matter how many directories I create (Jazz, Pop, Classical) it places all records in one directory in the order they were entered. The only control I have is to advance to next song or record (pretty bad). Because of it I use mostly "random" playback.
I already have DBpoweramp...thank you all for the great advice....
Do I need to rip the CDs again through DBpoweramp? I can'f just copy them from my existing library and burn the copy at 320k?
Yes, you can copy from the existing files. Within dBpoweramp there is a Batch Converter. You select the albums you want to convert, select the format you want them in, and select the destination drive and dynamic naming. And that's it.