Wheels / Tires: how does tire load index change with radial tire size, holding rim diam steady , say at 15 "

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Hello all--- tall traditional tires found on many Studebakers, say a 215 75R or 70R, ( both 15) , vs modern wide and short sidewalls---- vs load index--- any direct relation for load index to tire size ?? These new " performance" tires must see alot of load
on the fast moving hard turning cars, No ??