Spare Tire oddity

[Danishgod]

Why does Ram ship 18 inch spare tires after you spend a pile of money for a 20 inch rim and tires? and are there any issues to running 18 and 20 comingled together after you get a flat tire?

 

[CdnHO]

I suspect the circumference of the two tires is the same even though the rim sizes are different. Take a tape measure and measure across both tires and you will find it close.

 

[John Jensen]

The 20" rim center hole is too small for the spare tire carrier plate.
The 20" and 18" tires are basically the same diameter, so no issues running on the spare