Hi there is a very good video on youtube for just that
try search sketup to outerra or similar on youttube , i saw it other day , you will need to check out the <js script convention there is a thread here in forums too with a sport car setup , i think the script is v1.4 at moment
Edit found the video , basic priciples remain the same :) and the script thing must match wheel names with whats in the script and probably some other define like emitter names
http://forum.outerra.com/index.php?topic=1721.0 (http://forum.outerra.com/index.php?topic=1721.0)
Simple Wheels from Sketchup to Outerra (http://www.youtube.com/watch?v=0qafnUKaHTs#ws)