'80 Z28 tire sizes

I have a 1980 Z28 that originally came with the standard painted steel rally wheels. These had 225/70/15 size tires. I know this is the correct size for the type of wheel. I bought a set of OEM alloys and noticed afterward that the tire size is different (235/60/15's). They came off of an '85 Monte Carlo SS, so I'm wondering whether the tire size is because of the different car type. Anyone out there know the correct tire size for '80 Z28's with alloys?

Thanks,

Alan