They are 36" maybe 37". The weld on fittings I used were for a 36" diameter and they fit correctly. When I stacked them horizontally I spaced the support brackets to have ~12" from the top of the bottom tank to the bottom of the top tank to allow for connections between the two. Also the bottom tank is on 4" spacers to make connections on the bottom of that tank. So I agree that 85" would be too tight. Need about 90" unless you did those connections differently. View attachment 257037
Cross bracing was added to the steel supports and supports between tanks added since this picture was taken (before there are concerns voiced that they might fold over ;-)
Wow, slick setup. Is yours in a an outbuilding or a basement?