Spare tire okay to run?

Slavy

2017 2500 4x4 6 cyl. with 34,000 miles
Hi There,

I have a 2017 4x4 with T/A K02 tires - LT 235/85R16 on it. I had a flat the other day and put my spare on and it is a LT 245/75R16. I drove home for about 300 miles and had no problems. I took the bad tire in for a repair and they found a cracked wheel (posted on another thread about this). I ended up getting a new rim and got things back to where I was before. At the tire store when the guy was wheeling my spare and the other tire side by side, he noticed the height difference. (30.5" for K02 vs. 31.7" for the spare) and said I should not run them together.

Will running the spare hurt? Does it damage the differential if put on the back? Any issues with running it on the front?

I search the forum and could not find a thread on this. Thanks!

Recent photo from a road trip - Austin, Nevada area.
 

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Mr. Bills

Raconteur Emeritus
I would not run mismatched tires on the rear axle except to make a run to a tire store for repair or replacement.

A 235/85R16 tire has a diameter of 31.7 and makes 636 revolutions/mile. Your 245/75R16 spare has a diameter of only 30.5" and turns 662 revolutions per mile. That is a 3.8% difference.

At the risk of being overly simplistic, when running mismatched tires the rear differential will always be trying to adjust wheel speed of the two different sized tires as if the vehicle is constantly turning. The result is increased wear and tear and binding in extreme cases, particularly if on asphalt with good traction that prevents the tires from slipping.
 

Flagster

Active member
:rolleyes: It's a Jeep thing to have a mismatched tiny spare off your lifted build rig...I would match them up ASAP
 

Mr. Bills

Raconteur Emeritus
:rolleyes: It's a Jeep thing to have a mismatched tiny spare off your lifted build rig...I would match them up ASAP

I've been running "donut" spares on my jeeps for years - A BFG AT 33x9.50-15 on my CJ with 35x12.50-15 tires on the ground, and a 255/85R16 spare with the 315/75R16 tires on my LJ. Sometimes the weight and bulk of a full size spare is too much, hence a smaller spare.

Mismatched spares aren't a problem in the dirt where the tires can easily break traction and relieve stresses on the differential gears. [The California Four Wheel Drive Association specifies spares within 3" in diameter of the tires on the ground as required safety equipment for its events.]

Its on pavement where the problems occur.
 

Slavy

2017 2500 4x4 6 cyl. with 34,000 miles
Here is what the owners manual says - seems like it is okay to do it, but just slowly???

"When using a spare wheel of a different size, do
not exceed the maximum speed of 80 km/h."

So, I exceeded the speed and will work on getting the right sized spare.

Thanks for your input
 

Kajtek1

2015 3500 X long limo RV
Real tire guys compare tire rpm as that is what plays the role in real life.
Nominal size brings huge error margins as tires are made with pretty loose tolerances and I have seen the same manufacturer making 2 different model tires in the same size, while having 5% difference in rpm
Since you drove for long time on mismatched tires and traction light did not come on, that means it is within acceptable rpms.
Sometimes cruise control quitting above 60 mph is 1st indication.
Spare wheel on my 2015 model is exactly the same what other 4. It only lacked TPMS. :thumbdown:
 

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