2 ways to do this :
first one : the terragen extremist way (the slow-hard one).
Export your cars with differents objects. (one for each wheel and the other parts).
In terragen animate the wheel axis and use motion blur at render time.
In can be a good idea to animate the car in an another package. Something more classic like Maya, blender, 3ds,xsi, etc...
Terragen is not very suitable for this kind of job.
It can be easier to animate the car in your 3d package then export .obj sequence to Terragen
the second one : the easier way to me
For static purpose :
Some postwork on wheel on Photoshop and Gimp can give some very good result. Try to mix different kind of blur with differents opacity, use smudge tools, water drop tools this kind of stuff
For animated purpose or static.
There's some very good tools to recreate motion blur on 3D render picture after render time.
You can find this kind of tool in every good compositing package as plugin or build-in feature.
One i like is Real Smart Motion Blur
http://www.revisionfx.com/products/rsmb/.This tool track every pixel in the source video and try to create motion blur.
With this technique you can have some very good result without loosing time to calculate motion blured picture in terragen.
