Diameter of 8x17.5 tires

[tomr]

...showing my ignorance again, but how would one determine (without measuring) the diameter of these 8x17.5R tires? I've tried searching this, and as near as I can tell, the assumption is that these tires have an 80% or so aspect ratio. So, if the 8 is the tread width in inches then 80% of that would be 6.4 inches. Doubling that to make up for each "side" equals 12.8". Add to that the 17.5 inch rim size, and you have a diameter of 12.8+17.5 = 30.3 inches. Does this sound correct, or am I missing something obvious ?
Thanks
Tom

[Elandan2]

Tom,  You can find that information on the manufacturer's website usually on the load and inflation table pages.  Rick