I picked up my spare tire "donut" from my dealer today, they took two weeks to research it and get it in stock. Cost was $255.00 It's a T16x4T wheel and the tire is T125/80/D16 97M. The overall size of it measures 24 inches and the tire that is on my car now measures about 25.5 inches. I thought it was important to maintain the overall height on all 4 wheels.
I'm not quite sure I understand the math on that tire combo set. It would seem you are 1.50 inches different in height, that's a Big difference. Feel free to correct me if i'm wrong.
The way I'm reading this, is you got the Spare Donut Rim and Tire seperate and have yet to have it mounted and Bal.
So the 'rest of the story' ( as Paul Harvey Radio would say) will be after you get it mounted & Bal. to see if it all fits and works well with the Brakes and the Other Original 18" tires, correct.
Thanks for keepping us all in the loop of Info.