Thanks Mechainiac, the link has just the info I needed. I went to 3 different tire stores and none of them were right. Two of them admitted they were guessing. One of them stated with convincing authority, "The closest to 7.50 x 16 is the 215/85/R16. It's only 1/4 inch difference in diameter." Accoring to the chart you referenced there is a full inch difference. It's settled, the 7.50 x 16 tire size that originally came on a 1978 GMC 1 ton Dually Crew Cab, converts to a 235/85/R16!
