I still don't get how the Torque (engine_force?) at some point maxes out the speed. My programming is currently done with "dummy" proofing. I was never keen on the whole maths and physics things, although I do understand most of it with the right visual learning materials.
So if I understand correctly, you are just using the radius on a single axis of what @Megalomaniak circle graph shows?
I just have an if (fwd_mps < max_speed): give value to engine_force , else: engine_force = 0. And to counteract the problems of low speeds on uphills, I'm lerping it: if (fwd_mpsf < max_speed): engine_force = lerp(max_torque, engine_force_value, speed_scale), where max_torque is just the engine_force_value (being an export value) multiplied by 2.
So currently from 900 to 450.
These are the settings of my VehicleBody:

(inertia isn't used at the moment, it's still "leftover-code" I used when I wasn't able to get interactable collisions to work)
While uploading this image, I noticed that some of my values on this node are different from the same properties inside the node. So which one does it follow actually?
On the node (image), I have a weight of 1300, yet when I go inside this node, I have a value of 1000.
Also for the Can Sleep, on the node, it isn't checked, but inside the node, it is checked.
And these are my VehicleWheel settings:

So my Friction Slip was first at default value: 10.5, with the rear wheels slightly offset as recommended to get a drift. But that gives unwanted results. Even values below 3 don't give any satisfying results. So with your recommendation around 3, I got it to work. I used 3.5 for now with the rear slightly offset.
It works, kinda, but still not how I want/need it to. But it's a tiny step forward atleast.
Since my game in some ways relies on drifts and drafts, I'll probably have to try different approaches. I need more control of the drift (unless I want them to not have full control over it, like near an explosion) and I also need to be able to track such things, to build up a meter that you can use to trigger stuff or to boost.
For the collider/collision of my car, I just brought the collisionshape (just a box) down a bit, to the floor actually. It apparently also helps to keep the car stiff and not so bouncy. I tried bringing the car down "the floor" like you did, but then I would get a slightly wobbly car again.

It's nice to see a few other 3d racers over here, so that I'm not feeling completely lost :# :)