I found this while surfing, thought I’d pass it along. It has alternate tire sizes from 14-20” wheels, and tells you how close in size to oem, in order to keep your speedometer accurate. http://www.automobile-catalog.com/tire/1971/85565/buick_gs_350_sport_coupe_4-speed.html
Yeah, made me laugh too. 275’s are not “very” wide in my opinion. My daily runs them at all 4 corners, haha!