I bought my 2500 6.4 cc 4x4 last week. Got the the blackout package with the 20" wheels...
I just found out my spare is the stock 18" wheel. WTF?
My dealer said and showed me the spec sheet as built at the factory...on my vin.
it shows the blackout, 20" wheels, and.... 18" spare.
there is a .9" difference in size and that causes the 18" to turn faster than the 20. So, you and I will have to swap the spare with a front wheel in a rear flat scenario.

 

Does the owners' manual say you can't run the spare on the rear? .9" difference in diameter is more than I'd want to see from an aesthetic point of view, but the diff doesn't care.

The only functional issue I can see is that the smaller tire will have more power available and therefore be more apt to lose traction first. But I'd say the tread would have far more effect than the extra leverage from the slightly smaller diameter.

 

Agreed. That is not a big enough difference to matter for the short time a spare is on. If you want to do a two wheel swap when you get a flat....have it but it's not necessary. Hell, I'm sure that people are running mismatched air pressures that will cause that much difference in dia.