Inquiry: Mismatched rear tires within .25" tolerance?

[Mr Dubya]

A good friend just replaced a flat tire on the highway today. Rear right.

Soon after, he felt the drivetrain jolt proportionally faster as he gave it gas. He suspected either the tranny, t-case, or front diff were giving him trouble. Suggested that the 4WD was kicking in and out.

Got a tow and brought it to a shop. The mechanic said it was because his spare tire was only slightly larger than the drivers side, and the slippage caused the 4WD to kick in and out.

They replaced the tire with what I believe to be the right sized tire and the problem ended. I buy this, and have no reason to disagree with it, however:

1) The mechanic made a statement that even a tire that was a quarter inch larger could mess up the traction and differential's response.

2) Am I wrong in believing this wouldn't be enough of a difference to cause a problem? Is not a differential's very design to allow two wheels to travel at slightly different speeds?

3) And lastly, is there THAT tight of a tolerance than even a spare with .25" more tread (.125" each side mind you) would throw off normal operation?


Looking to be educated on this, thanks.

 

[D Hook]

From what I've read on this site and experienced myself, yes, that's enough to make the 4x4 kick in. Best to pull the fuses to the auto 4wd when this comes up, until you can get a new tire.

Some people buy a new tire when replacing the set but I don't know if this solves it without adding the spare to your rotation schedule so it has the same wear as the others.

 

[obie]

With AWD, or Auto 4WD, 2/32" to 3/32" (.09") difference in tread depth is enough to cause problems. Differentials are designed to allow for different speeds during turns and short terms, not constantly engaged. That leads to heat build up, increased wear, and eventually gear or bearing failure.

Edit: I just pulled up a quick article from google to give you a bit of information. http://www.dunntireblog.com/tread-depth-tire-replacement-4wd-awd-vehicles

 

[Mr Dubya]

Fantastic. Just what I was looking for, thanks obie.

One last thought - if one of you were to run in to this, and needed a replacement tire of the exact same diameter, how would you go about that? Measure every tire at a local dealer or junkyard? Seems a bit of a predicament to have three other tires useless when one isn't compatible.

 

[HiImBrian]

Fantastic. Just what I was looking for, thanks obie.

One last thought - if one of you were to run in to this, and needed a replacement tire of the exact same diameter, how would you go about that? Measure every tire at a local dealer or junkyard? Seems a bit of a predicament to have three other tires useless when one isn't compatible.

Do a search for the Brown Wire Mod and do it. It basically gives you the ability to switch to straight 2WD. Then put the "slightly" larger tire up front. That should solve your problem; at least temporarily.

 

[Mr Dubya]

Do a search for the Brown Wire Mod and do it. It basically gives you the ability to switch to straight 2WD. Then put the "slightly" larger tire up front. That should solve your problem; at least temporarily.

My thoughts exactly. I think that's now on his priority list of mods

Thank you gents!