I use several sources, Denny. One is a copy of the Old Timers Tire Guide, which has a couple of conversion charts in it. One converts from the old style system (6.70-15) to the Alpha-Numeric system (F-78-15) and P-Metric (215/75/15). It also has other sidewall ratios for the Alpha-Numeric and P-Metric sizes. I also use Coker tire to get dimensions for any tire I don't have listed, or want to get the approximate size, then I cross reference them with the Ultimate Tire and Wheel Fitment Guide, which is more for doing plus sizing on modern cars and trucks, giving suggested tire sizes, oe bolt patterns and offsets. But it also has a very extensive P-Metric, LT-Metric, and floatation tire dimension chart.
