This isn’t an answer, but maybe this tutorial will help (it helped me): Rig and Drive your first Chaos Vehicle - Unreal Engine 4 Tutorial - YouTube
(Specifically the step at 16:50 (create an external torque curve) was something I initially skipped, assuming the curve drawn in the blueprint details would suffice. But after hitting that create external button and saving it as a separate asset, my car was able to drive 
In particular, I used the 4.26 Chaos Preview 2 build from the Epic launcher, and activated the Chaos Vehicle plugin in my project. I also made a new test vehicle from scratch in Blender (just 4 cylinders and a simple boxy body) instead of a pre-rigged PhysX vehicle I had lying around (which didn’t move for reasons I could never determine). Not sure what I did differently, but going through all the steps from scratch more than once helped, after which I converted my existing PhysX project successfully.
Note that the Chaos Vehicle implementation (at least in blueprints) still has significant issues, so just treat this early preview version as a test playground to find and report bugs and get a taste of what’s to come.
If it’s any consolation, I find the vehicle movement looks/feels more natural than PhysX, and doesn’t seem to suffer from some of the weird bugs in PhysX (shooting off randomly into space)… so worth the wait even if you don’t get it working right away
Good luck!