J Lo was spotted Wednesday testing out her new whip -- which looks like a 2019 Porsche 911 GT3 -- with cameras rolling in Miami, and A-Rod standing nearby making sure everything went off without a hitch.

Gotta say, the fire red ride suits Jennifer well -- hot, sexy, classic ... just like its owner!!!

Meanwhile, it  looks like A-Rod was inspecting the gift upon delivery ... which showed up with a giant yellow ribbon on the hood. Nothing less than perfect for your honey, right?

J Lo is celebrating the big 5-0 at a shindig on Star Island -- just off the coast of Miami Beach. Seems like it's gonna be a giant birthday bash too, considering what was being brought in and all the prep work that was going into getting the place set up.

You can see giant golden orbs that are hanging all around a bunch of gazebos on the waterfront. Ya gotta assume J Lo's gonna go all out for her big day. And, rightfully so, of course ... it's a major milestone for her storied life.