I think you should try a product called Shoe-Goo (available at almost any department store and a lot of hardware stores). It will bond rubber or plastic to almost anything.
I have used it for many many things, the most rigorous being when I used Shoe-Goo to bond a set of plastic toe sliders to a pair of motorcycle roadracing boots (cornering lean angles sometimes result in the toes of boots contacting pavement, and the sliders protect your expensive leather boots). The sliders stayed on until they wore all the way through; without the Shoo Goo failing. Unless you plan on subjecting your 12C's new feet to sliding on pavement at speeds GREATER then 120 mph I think it will work fine.
I have also used it for applications similar to yours, and I think the main advantage it offers over other adhesives is that it never completely hardens. While it has high enough shear strength for the application I mentioned above, the "Shoe-Gooed" item CAN be removed by carefully prying it up away from the item to which it is glued (maybe Shoe goo is weaker in tension than in shear). So, if you ever needed to, you could get to the screws and disassemble the calculator. The excess Shoe Goo can then be "rolled" off the calculator and the rubber feet with your fingers and reapplied when you reassemble the calculator.
By the way, my 11C needs new feet and I think I'll use your "tire" idea and some Shoe Goo to accomplish it. Thanks for the idea.
Take care.
Wayne
Edited: 3 Mar 2004, 7:48 a.m.