Not trying to sound like Im brushing you off but take a good read here.
Wheel tech:
http://www.tirerack.com/wheels/tech/index.jsp
Tire tech:
http://www.tirerack.com/about/techcenter.jsp
There are really some very good tech articles that will help you with the overall tire and wheel fitment issues. Including the nomenclature used when dealing with wheels and tires. Some great info..
For your Q. The 20 you say is what, 20mm. Pos or neg? Read the offset tech blurb.
http://www.tirerack.com/wheels/tech/...jsp?techid=101 and it may help. JR