I think that both of the sets of the M/T Street radials tires I ran(295x65x15, 315x60x15) had the same circumference when measure with a seamstress tape, but maybe not as that was a long time ago
I have 96.0 inch circumference for both in my mind, the 315x60x15 on a ten inch wide rim and the 295x65x15 on the 12 inch wide rims
I ran 4.10 rear gears in the car most of the time and I would see a 500 RPM drop switching from the 315x60x15 E.T. Street radials to the larger diameter 10.5x31x15W M/T slicks which had right at 102 inch circumference on the ten inch rims, no real MPH change in the 1/4 mile though that I remember, just RPM recorded on the VDO playback tach in the car