About twenty years ago, I was working with a former BF Goodrich engineer, and the topic of reproduction vintage tires came up. I felt that having a tire made from an original mold was no guarantee the carcass that went into the mold met original quality. Also, suppliers didn't always offer the size that I was looking for.
(Mis always an issue, too.) He saw nothing wrong with using modern radial tires on my old cars, but I was concerned about tire circumference - wanting to avoid recalibrating the speedometer or swapping the pencil gear. He then showed me how to do the math on bias-ply vs. metric radial sizes to get the best match.
When it came to my V8 Packards, it looked like a 235/75R15 would replace the 8.00x15 (later, an 8.20x 15) tire on a 15x6" wheel, as used on 1955-56 Senior Packards. It was a close call for the 7.60x15 on a 15x5.5" wheel, as used on 1955-56 Junior Packards (Clippers) and the 1956 Executive, but I prefer the 225/75R15 for those cars/rims.
The use of these radial sizes seems to be supported by Coker Tire's conversion chart:
http://resources.coker.com/tire-tech/tire-installation/bias-ply-to-radial-conversion-chart.html I did find, however, that I had to let some air out of the 235/75R15s to get them to fit between the brake drum and the wheel opening of one of the rear fenders (even with skirts removed) on my Caribbean Hardtop.
I don't know the bolt circle off-hand, but they are different on Junior vs. Senior cars.
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