When you go up an alloy size, or two!!!, you put lower profile tyres on so the overall diameter, and therefore circumference stays the same.
I have 45 profile tyres on 17's with a width of 215, I assume yours are 35? or less (total guess)
To get the overall diameter:
The profile figure is the tyre wall height as a percentage of the width, so in my case my tyre wall = 215 x (45/100), then for the diameter, double that (to get the tyre wall top and bottom) and add the wheel diameter, converted to mm, so 17x25.4
Giving a diameter of 2x(215 x 0.45) + (17 x 25.4), in mm.
Carry out the calculation on your wheels, and again on the spare and see what you get. I haven't done it for mine but I can see that my spare is miles out!!! Be interested to see what you get