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I was able to successfully cut through some old tires using a 7.25 inch circular saw, albeit one fitted with a relatively high-end blade (a 40-tooth 'Diablo' woodworking blade). There are blades designed specifically for cutting through metal, as well, but they're even more costly than the Diablo.
In any case, I was able to cut through the tires without too much difficulty using the circular saw. I had to cut across one side, flip the tire over and cut across the other, and then stand the tire up and cut across the tread in order to get the tire cleanly in half. The blade seemed to hold up well enough, as it cut through the fourth tire about as easily as it cut through the first one.
This is probably very hard on the saw though. I Wouldn't recommend it if you have more than a few tires to cut through. And I'm guessing it may not work at all using the cheap blades that come standard on most circular saws.
Also sparks will go flying everywhere when you do this. Eye-protection is a good idea.
By aroth 
Have you tried Paul Farbers way of cutting and turning tires? It's actually easy and kinda fun.
Editor's Note: We found this site that talks about Paul Farber's methods: http://www.touchtheearthranch.com/goatshed.htm
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Here are questions related to Cutting Tires.
There are plenty of ideas on the web, but few if any on how to make these designs. I just want to cut the old tires in half to make a path border, but am having a heck of time finding out how to cut them.
Ideas I have got, how tos I don't.
By Tim s.
How do you cut old used junk tires in half so they can be worked with?
By Robert S. from Jacksonville, FL
How to cut up discarded tires?
By Joe from San Antonio, TX
By Rob 09/04/2010
Large tin-snips/metal shears.
Thrifty Fun has been around so long that many of our pages have been reset several times. Archives are older versions of the page and the feedback that was provided then.
Could you inform me on how you cut car tires please?
Debra from NSW
Incidentally, unfortunately it's not a good business for the Africans because the sandals actually last multiple human lifetimes so they don't get any second-time customers. I guess they need a lesson in planned obsolescence. (07/12/2006)