19.5 Tire Size

[soxplayer]

My wife's 67 C20 is an old Forest Service Stepside with ultility boxes. The GVWR is 7500 and according to the owners manual it came equipped with 8-19.5 6 ply front tires and 8-19.5 10 ply tires in the rear. Right now it is on 7.50-16 LT on demountable 8 lug rims. Does anybody now how to determine the diameter of the original tires (i.e. 78,80 or 90 series). The differential is a 4.56:1 and the shorter tires really limit highway travel. I am considereing 235/85 16 on newer tubeless rims but I really want to know how close these taller tires are to the original diameter in order to lower the highway revs to an appropriate level.