KEY SMARTKeySmart Nano Clip - Pocket Clip Key Ring Holder - Secure Your Key Chain, Eliminates Pocket Bulge

Q: How well does this work with car keys? I'm thinking about this for my girlfriend because she'll often misplace her car keys when she is here.

Just slip it on her purse or pants pocket and the keys will be there when you need them.

Q: Can this product hold tight on pants and won’t let go?

I've used them for running, and I clip on my key to carry. It is well suited to hanging the key inside your pocket, with the clip both attached to the inside and out. I have clipped them onto the waist band of a pair of pocketless shorts. The best scenario will be one where the clip part is over a seam, cuff, or hemmed edge. That way it opens up a little and closed back up as you push it past the "seam" bump. BUT I would not count on them to not slide off a piece of fabric without an edge. They are not vice tight for sure.