If you have a 4x4 vehicle with a TracLock (or any type of posi-traction unit) rear differential carrier, you should never run different diameter size tires on the same axle.
It could wear out the clutches in the carrier very quickly.
Another sound reason for having a spare tire the same size as the other four tires.
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05 Jeep Liberty CRD Limited 
Ironman Springs/Bilstein's
Yeti Stg4 MaxTune
Weeks ElbowKit/BatteryTray
NO FCV/EGR
Samcos/ProVent
SunCoast/Transgo
Carter Intank-pump
2mic.Sec.Fuel Filter
Flowmaster/NOcat
V6 Airbox
GM11 blade fan/NAPA HDclutch
Ironrock Trilink
Dodge Cummins, 5"exh. Viair, Huge K&N