You increased Tire width and cross-section height, while keeping the same rim diameter, there is no way they are the same diameter.
You increased overall tire diameter by 2.07" or 7%.
Keep in mind these are calculation of the nominal tire size listing, the actual overall tire diameter will vary and should be listed in the detailed specs by the manufacturer. Its safe to say though, there is enough difference between these two tire sizes, there is no way the typical manufacturer variance is going to make them close to equal. The most valuable information is the Number of Rotations per Mile, that is an actual tested figure to tell how much different the rotation speed of the tire will be compared to another.
If you have 4WD, you have to have all 4 tires spin at the same speed when driving straight. If the tires spin at different speeds, the differentials will have to rotate internally to make up the speed differences, just like when you make a turn. Keep in mind, with the intended configuration of all 4 tires at the exact same diameter, the differential may rotate internally 10% of the time, with one tire a different diameter, the differential will rotate internally 100% of the time. You will wear out XFR case first and then axle differentials.
Why do you think the O.M. recommends rotating tires every Oil Change? To keep the tire wear even enough that it doesn't create stress on the differentials, especially the XFR Case Differential. On another forum, I had a former Jeep Engineer say that simply driving on 4 of the same tires, but the half the tires at the tread bars and the other half brand new tread depth, front/rear, is enough difference to wear out the XFR Case differential.
Yes, if you use the smaller spare with 3 larger tires, the ABS/Traction Control/SCS/BAS/etc is going to recognize the difference in tire rpm, it likely will react unpredictably and undesirably. But the stress on the differentials is worse IMO. Once you get the tire fixed and go back to the same diameter for all 4, the ABS/Traction Control/SCS/BAS/etc will go back to normal no damage. Meanwhile, depending on how long and how fast you drove with one tire significantly different in diameter from the others, you may have significant wear, significant reduction in the life of your transfer case.
If you don't get a spare the same diameter as the other tires, and you have to use it, I would only drive very slowly a few miles, to get the tire repaired, otherwise you're risking the XFR case.
TPMS, yes Commanders have 2 different systems, Premium that displays the pressures of the 4 tires in their relative positions, and Base that will only light a warning light if something is wrong with the system or one or more tires is below acceptable pressure.
I have the Base TPMS system, my spare has NO TPMS sensor, that has never caused any problem at all in my 2010 Commander. (I bought my Commander used and missing a spare, that was replaced by ordering the cheapest steel wheel that fit using an OEM tire in stock, so I have no idea if the Base TPMS system spare came with a TPMS sensor or NOT).
I've seen posts from people with the Premium TPMS system talking about the Spare Tire and its sensing the presence and condition of the spare, as well as having problems because of TPMS sensors in the spare.
I was assuming the Premium TPMS includes monitoring the spare and the Base TPMS does NOT monitor the spare. On this thread we have posts from folks that say they have the Base system and get puzzling reaction to do with the spare. So, I'm at a loss, NOT sure how the spare plays into the Base system, perhaps they changed it over the model years. I have a 2010, from all my TPMS interactions, it appears the 2010 Base TPMS system does NOT monitor the spare tire.
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Last edited by Mongo; 03-24-2016 at 08:40 AM.