I agree. Changing the rims changes the inside diameter of the tires. But the outside diameter is determined by the other tire parameters.
I'm not sure how to cross the tire size to get from one rim to the other. For example (out of thin air) suppose you have a P235-70R16, the 18" cross isn't just swapping 18 where the 16 is. One or both of the other numbers will change as well in order to maintain the same outside diameter.
The numbers involve ratios rather than exact dimensions, and I always have to look them up. Whoever came up with system of tire designations was just a little different, I think
That said, given the actual designation for the stock 16" tires, it shouldn't be too difficult crossing them to an equivalent 18".
Doug
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