I assume the front and rear rims are different.
Which rim does the factory spare have?

Also, someone explain the theory of dually tire rotation to me.

thanks.

 

actually (at least on my truck) all the rims are the same..when i bought it, it had brand new tires on the front and outer rears..the inner rears did not match at all ...so i bought new tires for the front and just had the inner rears changed and the new tires went on those rims and the orig new matching set went on the rear..there all the same, and also with a duallie you dont get a spare..you have 2 spares in the rear already..if you get a flat front you just take a rear off and use that and go with 1 rear for awhile 'till you repair/replace the flat tire
...zap!

 

rotation...the front tires you swap with the inner rears...and there you have it!
...zap!

 

^^^ correct. That is why the front wheels look ugly on a dually.

 

A spare tire is a build option, not just on the dually but all Super Duty's.
If the truck was ordered with it, it will have one, if no, no spare.