"you may see the front wheels has half intersect with ground surface..."
Hmm...i have to correct you here, Madman.
The fact front in-cockpit tyres are half cut isn't due to the height of the cockpit, but only to a visual effect. Because cockpit is at the right place but GP4 Designer's only created half in-cockpit front wheels to save ressources as you can see if you open the car_wheel_front_cockpit.gp4 file with my 3D Editor. Your eye just think it's half cut due to intersect with ground, helped to it by the lack of wheel shadows.
Else, steering wheel is a shape object of course, it's the cockpit_wheel.gp4 file stored in the cockpit folder inside cars.wad.
You will be able to displace it with 3DEditor but it will be long as you will have to displace every component (leds, messages positions, etc...) one by one for now.
Just a hint: if you wish to see the full textured steering wheel, just put
new_steering_wheel_displays.tga
steering_wheel_centre.tga
steering_wheel_ends.tga
in the same place as the cockpit_wheel.gp4 file and open this file with 3DEditor.