Finding tire size with rim?

[87Aveo]

Hey everyone. So I had a very nice gentleman just drop off a set of 4 aveo rims (the black steel ones) off of his written off aveo. I heard him talking yesterday at my work saying his wife wrote off her aveo and he said the only thing not wrecked were the tires and rims haha. So I said oh I have an Aveo...yadda yadda yadda... one thing led to another and he was very nice to drop them off to me this morning for free! Now the thing is... my aveo now has.... 14's I think on it? Still using the stock black steel ones on an '04 hatchback. These new ones he gave me measure 15 inches in diameter by 7 inches wide. The ones I currently have are 14 inches in diameter and 7 inches wide. How would I figure out the tire size for these new ones so that I can get some new rubber for them?? Thanks!

Just remembered the last tires I bought on my current rims were 18560R14. So would the new ones be something like 18560R15 then? Thanks!