I've been doing some research and the consensus diameter for 8.00x14" seems to be 27.58". Many folks go with the 215 or 225R7514's. I went to the Coker site to confirm these. You can plug in the Year/Make/Model/Tire Size:
I also went to the Universal Vintage Tire site:
Coker 8.00x14 - 27.58"OD x 4.58"W
Coker 8.50x14 - 28.10"OD x 4.75"W
Universal 8.00x14 - 28.67"OD x 4.95"W (but there were varieties of 800-14 with different diameters).
I have 15" Rally Wheels so I have to figure out what tire sizes come close to the 27.58" diameter. I replaced my driven gear (18 tooth) recently and run about 5mph faster than actual speed up to 50 mph. After that, the differential gets larger as the small diameter turns faster than actual.
Frankly, I don't need new tires right now, but when I do, I will try to figure out the closest match for a 15" wheel.