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5.8' is 5.8 feet. So 5.8 x 12 inches = 69.6 inches.
5'8" is 5 feet 8 inches, so (5ft x12inches)+8 inches = 68 inches.
Per the new truck configurator a short bed on a 2021 is 69.92 inches, so about 5.8 ft.
A standard bed is 79.44 inches, so about 6.6 ft. Or roughly 6' 7".
It's easy to see why there is confusion, even among the marketing material from Chevrolet because the marketing people aren't the designers. The designers are going to use inches to be more precise, which then other people convert to feet because the average American can't easily visualize 69.92 inches, but you tell them in feet they will relate to it better.
So when you convert from inches do you just divide by 12 and say 5.8 ft, or divide by 12, then carry the remainder and convert back to inches? This is an example of why other countries use metric.