JB - Welcome back sir! According to the GM Heritage Center 1961 Chevrolet Truck Data PDF the screenshot below shows the different specifications for the C/K/P10 trucks depending on the Base or designated RPO for the truck.

Their where 3 different size options, 15", 16" and 17.5" depending on the Base or RPO option selected.

Note: All the 15" and 16" wheels were listed as 5" (5K) wide for the rim size. The 17.5" wheels where 5.25" wide.
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~ Dan
1951 Chevy 3 window 3100
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"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)