Wheel measurement question

Hey guys I'm doing steel wheels with dog dish caps on my dart I'm wanting to do 15s on the back and 14s on the front just because I already have 4 14s and I like the raked look and the 15 tire options on the back. I have one 15 it came off a 86 ram 2wd it was the spare it still has a tire on it so i can't get a measurement of the inside but the best I can tell the outside of the rim measures 6 inches I know your supposed to measure the inside of the rim so of the outside is 6 does that make this a 5 or 5.5 rim? Does anyone know of the top of there head the size of a 86 2wd spare wheel? The reason I ask is I found a really good deal on a nice 15x5.5 if they are the same I'm gonna buy it. I attached a photo of the one I have hope it helps

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