I used to work in a paint store and also as an auto detailer. You would be surprised how often your situation occurs. The first thing to know is if it is oil based or water based(latex or alkyd). If it is water based and dried you can simply use a paint rubbing compound to get it off the paint. Rubbing compounds basically cut through paint. Just don't kill it when you are using it, take your time, if you have access to a polishing wheel use it because it will help to stop you from concentrating on a specific spot and will feather out the cutting process. The same process will work on a oil based paint on the cars paint. For the wheel well (if it is oil based ) the is something called tolyuol( I think thats how it is spelled) pronounced tollywall. Just put some on a rag and rub until the paint is removed. If it is latex on the wheel well, especially if it is thick you should be able to just scrape the majority of off, and then just use soap and water to remove the rest. Most house paints take a lot time to cure, and consequently if the are not properly applied can be removed by some process even after months. Hope this helps if you have not figured it out yet. You may want to take it to a real good detailer too. They usually have a product for every posisible application.