You can start practicing now. Visit this website and take the previous preliminary tests. Then, check if your score is good enough or needs improvement.

Score of 23 or higher: good chance of passing. You are encouraged to proceed to the final examination.
Score 19-22: 50-50 chance
Score 18 or lower: poor chance of passing
http://www.lsa.umich.edu/umich/v/index.jsp?vgnextoid=9088d14864cb4110VgnVCM1000003d01010aRCRD&linkTypeBegin=contentlinkTypeEnd&vgnextchannel=3129d14864cb4110VgnVCM1000003d01010aRCRD&highlightChannelBegin=3129d14864cb4110VgnVCM1000003d01010aRCRDhighlightChannelEnd&assetNameBegin=Preliminary%20TestassetNameEnd