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thanks Dan! and yes Rod it's been a long nap and the trouble with looking online is 1 place says one thing and another will say something else. i went straight to Mickey Thompson and got 2 different answers from 2 different people. Trust me, ive tried to figure out what each number means. the last number is the only one that makes sense to me



That's because they aren't EXACT conversions.
Think of a word in English and another language. Many are direct but some, especially slang, do not translate literally.
The only reason you would need to convert the late 60s early 70s tire sizes is because you want an exact replacement for OE equipment. The metric radials are NOT exact replacements.
Also, darn near every tire shop will offer a recommended tire size according to application.
You want a Challenger R/T tire? They have that listed.
So if one says 225/75R15 and another says a 215/70R15, they are both close but one is the same height and fatter, the other is the same width but shorter.

If you have looked at any of the explainations of tire sizes, you can see that a 215 tire is much more descriptive than a G tire.

A 215 tire bulges out to 215mm across the width of the tire when mounted on the proper width rim and inflated to the proper pressure.
How wide is a G tire?

The second part is 70, 75, etc. This is the same on the older stuff and the new. A 100 tire would be as tall from the rim as it is wide. A 50 series tire will be half as tall as wide.

http://en.wikipedia.org/wiki/Tire_code