Try pounding on the tire instead of the rim. If you can't get it off with a few pounds from your hand then try kicking the tire with your foot at the 12 and 6 o'clock positions.

I use anti-seeze compound on the wheel hub to prevent the tire from "welding" onto the hub.

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles