Not all tires of the "same" size are created equal. The formula for computing theoretical tire size is quite simple but actual tire sizes may vary a bit.
Visit the websites of various manufacturers and search for tables. If none are available, the size calculators will have to suffice.
Examples:
http://www.mastercrafttires.com/html...ger_avenger_GT http://www.goodyeartires.com/eagle/eagleF1AS_sizes.html
Tire sizes are like shoe sizes. If all shoes were made to the exact same standard, we wouldn't have to wear our good socks to the shoe store.
Cheers!