I used TestPhysicsCar example, it has different method of adding wheels to a car, but at last result is same. Example here. I have been coding my own game so I studied both examples, Fancy Car and Physics Car and I combine it in one. Fancy Car is too difficult to run all things corectly for me, only experts could make it run right. In Fancy Car example you say: go here, find this, create this from it and do this with it, but in Physics Car you say: here you are, do this… In Physics Car you have to replace jMonkey geometry to your own models from blender and say where you want to place it. You have to make independent scenes for car model and wheel model.
I rework your Coupe_Green, I split car and wheels and put in new scenes: car, wheel_left and wheel_right. Wheels have to be placed in the centre (0, 0, 0) and good is to place car too in the centre, but with bottom of the car on 0 of Z (up) axis or higher. If you want to lower the centre of mass, just move the car up on Z axis in blender and increase restLenght of suspension in jME.
Here are your reworked project.
Here are docs for vehicle physics
I had a one little problem with your project, jME doesn’t like stateManager.attach(bulletAppState); so I copyied your project files to a new one created in my version of jME.
Ask a question if you are missing something, I will explain it.