Tire size and rear gear ratio in 64 B body

Now running a 27 inch tall 15 inch tire and was wondering what the actual rear end ratio is.If the ratios were calculated on original tire size then a larger tire will diminish gearing.So if the new tire combination is 10% larger than the original with a 3.23 rear gear would that make it more like a 2.94.This is assuming original tire was 24.3 inches does that sound correct.Don't know original tire size guessed at it.Anyone have info on original equipment tire sizes?

John

http://www.wallaceracing.com/Calculators.htm

I came up with approx. 26 inches for the 750x14 and 27inches for my 255/60/15. a 4% increase in diameter.So would that reduce the 3.23 rear end by 4% making it a 3.10 ratio with the larger tire.
Thanks for the site info.

John