What would be the stock tire size for a 1991 chevy silverado 1500.
Stock tires sizes are different for 2WD and 4WD vehicles, and they have changed greatly since 1991. Depending on the exact model and options the original tire size could have been 225-70, 235-70, 245-70, 225-78, 235-78, 245-78. Diameter would be 15 inches. There should be a label inside the drivers door that shows the stock tire size and what PSI they should be inflated to...but this label is often gone or painted over on a truck as old as yours.
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