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Old 11-06-2012, 02:41 PM   #1
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Running an oddball size spare on a 4x4

I’ve got a question maybe you guys can answer. My vehicle setup is four van tires + a spare and 2 trailer tires + a spare, all the same size. Since I had Deavers springs installed, my lift increased about 2”. Now my 285/75/R16 Toyos look way too small. Right now I have 6 Toyo’s…four with 25K miles on them and two brand new spares (one on the van…one on the trailer). The SMB trailer has the last BFG’s (285/75/16) which are getting too old. I usually pull my old vehicle tires and pick the best 2 for the trailer.

So I’d prefer to install four 315/75/16’s on the van but keep the two brand new existing 285/75/16’s as spares.

Here is the question… In a pinch, can I use a 285 as a spare to replace a flat 315 to get me to a shop for a new tire (which may be a long drive in some circumstances) without doing any harm? I have a LSD rear end and a air locker up front.

My plan would be to use the older existing 285 Toyo’s off the van for the trailer and always keep the spares as 285”s. This would save me a bunch in the end if it can be done. The old tires still have plenty of meat left and are good enough for the trailer.

BTW I can’t put larger tires under the trailer w/o modifications plus adding a 315 to the Aluminess bumper adds weight and I’d have to modify the tire mount as well.

Thoughts???
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Old 11-06-2012, 02:55 PM   #2
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Re: Running an oddball size spare on a 4x4

Perhaps others more knowledgable will chime in but I believe your fine for short distance at hwy speed as long as you run the smaller tire in the rear. It's more an issue in 4wd if it's on the front.
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Old 11-06-2012, 04:52 PM   #3
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Re: Running an oddball size spare on a 4x4

You don't want to run different sized tires on the LSD on pavement. If you have a rear flat, put the smaller spare on the front (leave it open) and put equal sized tires on the LSD. In the dirt, it doesn't matter much, 'cause the tires will slip anyway, but when you have traction the limited slip will both have to slip and add stress to the rest of the drivetrain. Oh, when you have traction, you want to be in 2wd also, for the same reasons.

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Old 11-06-2012, 05:20 PM   #4
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Re: Running an oddball size spare on a 4x4

That's how I also heard it cause it will eat the clutch plates in the diff but I just wondered if the difference between the two tires was mild how bad would it be
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Old 11-06-2012, 05:56 PM   #5
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Running an oddball size spare on a 4x4

It killed my diff in a pickup in about 20 miles running a smaller spare, but that was on the highway pulling a trailer. Hindsight is always 20/20. I won't risk it again.

Good luck!
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Old 01-28-2013, 06:03 PM   #6
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Re: Running an oddball size spare on a 4x4

To further ask about this, is there a difference if the tire height is only 0.3" in difference? I would think not but I don't know. Thanks
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Old 01-28-2013, 06:14 PM   #7
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Re: Running an oddball size spare on a 4x4

***mumble*mumble* buy the whole set *mumble*mumbledotdotdot
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Old 01-29-2013, 01:48 AM   #8
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Re: Running an oddball size spare on a 4x4

On pavement put always the smaller spare on the front and open the hubs. I did it often in the past (35" to 31") It works great and also driving with the small tyre in front is no problem.
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Old 01-29-2013, 09:17 AM   #9
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Re: Running an oddball size spare on a 4x4

Using a tire calculator, a 315/75-16 tire shows 601 revolutions/mile. A 285/75-16 tire shows 633 revolutions/mile. The calculator also shows a 5.12% diameter difference. As pointed out earlier this could ruin/wear out a LSD.

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Old 01-29-2013, 02:07 PM   #10
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Re: Running an oddball size spare on a 4x4

Quote:
Originally Posted by my3kids
Using a tire calculator, a 315/75-16 tire shows 601 revolutions/mile. A 285/75-16 tire shows 633 revolutions/mile. The calculator also shows a 5.12% diameter difference. As pointed out earlier this could ruin/wear out a LSD.

Ray
Before getting too concerned, calculate the difference in distance of going around, say, a freeway cloverleaf on/off ramp; or winding down a mountain switchback road - lots of differentiation in both scenarios.
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