anything 5 or 6 lug is typically 10.5" cap. My 77 c10 has 16" 5 lug wheels so the outside diameter means nothing as to cap attachment. 3/4 and 1 ton use a larger diameter cap due to larger bolt pattern. Any passenger car or s10 or light series truck uses the same cap, the stock 14" steelies from my 80 camaro could use this style cap but look awfully funny doing it. They also made some 16.5 6 lug wheels, very few, and some 17.5s as well.
If your wanting caps for your truck, fire the 5 star is for 1/2 ton in this style cap, the 6 star is 3/4 ton.
And no adam, your rallies and wagons probably dont have the outside clips for this style cap, only the stock steelies do. If you want to run this and keep your truck a 6 lug find some m1009 steelies.