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Old 01-29-2013, 02:57 PM   #11
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Re: Running an oddball size spare on a 4x4

Got me thinking, so I did some math. If you drove in a 300' diameter circle (typical diameter of a 40 to 45 mph cloverleaf) for one mile, you would go around it 5.6 times. Inside tire would travel 5280 ft (1 mile) and the outside tire would travel 5403 ft, or 123 ft more. If a 315/75-16 tire shows 601 revolutions/mile, then the inside would do 601 revolutions and the outside would do 615. Obviously the tighter the circle the greater the difference traveled between the two tires. On a 100 ft circle (tight mountain road) the difference is 42 revolutions per mile - 370 ft.

Does any of this really matter.

If you have a mismatched spare - best to run it on the front in 2-Wheel Drive, or on the rear in 4-Wheel Drive, keep your speeds down - and get the flat repaired or replaced as soon as possible.
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Old 01-29-2013, 03:36 PM   #12
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Re: Running an oddball size spare on a 4x4

Quote:
Originally Posted by BajaSportsmobile
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.
The difference is how far you drive around a ramp or corner - not the miles you would do on a highway.

Mike
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Old 01-29-2013, 05:37 PM   #13
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Re: Running an oddball size spare on a 4x4

pssst*buy a set*
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Old 01-29-2013, 05:51 PM   #14
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Re: Running an oddball size spare on a 4x4

I'm with Marc.

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Old 01-29-2013, 07:59 PM   #15
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Re: Running an oddball size spare on a 4x4

So, to run my new 315 spare on the vehicle, provided the rear goes flat I'd have to Pull one in the front, move it to the back, put the spare on the front and go happily on my way??? Wow never saw that one coming at me but I don't know much about this. Guess I should have saved the money on the 5th 315 and just kept one on the 285's for the spare cause I'm gonna have to flop the tires around anyway. What if one tire blows and I need a new one and the other 3 315's have 1/2 of their life left? I gotta buy 3 more tires to match the new one? I understand the 285 vs 315 deal; I went out and bought 5 315's...that extra tire was bad enough-ouch.
Daryl was asking about .3 of an inch??? I got to be missing something here
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Old 01-29-2013, 11:03 PM   #16
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Re: Running an oddball size spare on a 4x4

Quote:
Originally Posted by daveb
Daryl was asking about .3 of an inch??? I got to be missing something here
Forget about my post. After doing the "MATH" I see the diffrerence in circumfrence is less than an inch for the difference of .3". I should have realized that before I posted. Just so you know my 2 tires were 275/70R18 (summer M/T's) and 255/85R16 (winter studded). Didn't mean to confuse anyone.
Cheers
Darryl
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