As long as the total diameter on your NEW wheel+ tires is equal to the total of your STOCK wheel+tires there is no need to readjust the speedometer. It's calculated by the total number of rotations your wheels make. So by the two being equal, there will be an equal number or rotations.
As for tire choice and wheel choice..when I got mine, i was not very familiar with the inner workings and the technological aspect of cars so I didn't understand much. But it really helps to go to a dealer or someone like that who can help you choose the correct tire to match the rim to keep the same diameter.
Sorry, but I don't like the rim choice :\ and I wouldn't go with 20's. I think 19's is the biggest you should go...you should read through a few threads on the forum regarding this issue or any other issue you have in the future. It helps out a lot and it also gives you a lot of info prior to asking further questions. And trust...you really want to stay away from asking easy and simple questions lol...people call you out...sometimes...
