OK, right now I have 13" rims on my 90 Hyundai excel, can I just buy some 15"s and slap them on, or do I have to do some mods? :confused:

You have to check offset, rim size for your car. Basically check to see if 15's will fit your car. Tirerack.com has a good program to see what size will fit your car. Give them a check.

Since you will be plus sizing.. make sure you try to use the proper size tire that will equal (or come close to) the stock size tize. If you dont you can through your speedo off bad and could other problems such as rubbing, etc.