Hi Bastien,
I've never tried this but here's how I think you could do it:
Create your wheel model so that the origin is at the center of the wheel. When you animate the wheel you'll want it to rotate around its centre. In TG2 rotation happens around the origin of the object but you can't change the origin in TG2 so you will have to make sure the origin is in the right place before you import the model.
To turn the wheel you need to animate the rotation of the wheel object. I'm not actually sure about this but I think the axis of rotation will depend upon the direction you want to roll the wheel in. For example if you were moving it along the X axis you would rotate animate the rotation around the Z axis of the wheel.
I guess to animate the rotation you would need to decide how quickly you want the the wheel to turn. Lets say you wanted it to turn a full circle every 20 frames. You could set a key with rotation at 0, then 20 frames later set a key with the rotation at 360. Then for every 20 frames you would need to set a key with the value increased by 360 i.e. 720, 1080 etc. This is so that the wheel keeps rotating in the same direction. That's the easiest way, there is a way you could work it out so you only need to add another key at the end of animation.
Moving the wheel is pretty simple, just animate it in the direction you want it to go. You would want to match the movement speed to the rotation speed so it didn't look like the wheel was spinning too fast or too slowly.
The good news is within the near future the animation part of this will get quite a bit easier :-).
Regards,
Jo