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Old 06-11-2006, 11:49 AM   #1
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I upgraded my 235/80/22.5 Michelin XRV tires to 275/70/22.5 Michelin XZA1's last year...Great ride on the new tires...I want to carry an unmounted spare but only the 235 tire will fit in the storage compartment.
The diameter of the 235 is 37.1 and the RPM is 556. The 275 diameter is 38 and the RPM is 545. I know in a jam I could put the 235 spare on either front and move a front 275 to the rea if a rear blew but my question is could I put the 235 on the rear for an emergency.?
I read in a tire book that the diameter diff should not be more the .25 inches and the circumference difference not more that .75 inches...
I would appreciate any advice from those who know more about tire sizes that I do...thanks...RKL
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Old 06-11-2006, 11:49 AM   #2
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I upgraded my 235/80/22.5 Michelin XRV tires to 275/70/22.5 Michelin XZA1's last year...Great ride on the new tires...I want to carry an unmounted spare but only the 235 tire will fit in the storage compartment.
The diameter of the 235 is 37.1 and the RPM is 556. The 275 diameter is 38 and the RPM is 545. I know in a jam I could put the 235 spare on either front and move a front 275 to the rea if a rear blew but my question is could I put the 235 on the rear for an emergency.?
I read in a tire book that the diameter diff should not be more the .25 inches and the circumference difference not more that .75 inches...
I would appreciate any advice from those who know more about tire sizes that I do...thanks...RKL
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Old 06-11-2006, 04:44 PM   #3
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Good question and I'll be watching for an answer from a [hopefully] knowledgeable person.

But I'm sure there is no black and white answer. You can always put a mismatched tire on a wheel and "limp" along. Heck, you can drive on a rim alone if you really have to. So the question will be, how much of a "limp" will you tolerate with this spare on? If they say,"yes, the 2235 spare is OK if you keep the speed under 30 mph", is that OK? Or are you expecting to continue zipping down the interstate and complete the 400 mile drive home? Hugely different questions, I think.
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Old 06-11-2006, 04:45 PM   #4
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Get the same size tire you already have and just put enough air in it to keep it inflated but not fully aired up and see if it fits in spare holder. You can finish inflation when you need to install it with a small compressor or the tire service you call can inflate it when they get there to change it.
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Old 06-12-2006, 02:21 PM   #5
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You cannot run mismatched tires in a pair. The taller tire will take all the load and will, in all likelyhood, destroy the tire in a short distance. That much difference on the front axle will give you wierd handling too.

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Old 06-13-2006, 05:17 AM   #6
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">
I read in a tire book that the diameter diff should not be more the .25 inches and the circumference difference not more that .75 inches...
RKL </div></BLOCKQUOTE>

The tire book says no more than .75 inches and you want to put on a tire with .90 inch difference. Sounds like you have your answer.
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Old 06-13-2006, 06:11 AM   #7
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Carry one of the old size and if you have a rear flat move a front tire to the rear to maintain the same size and put the spare on the front until you can get the correct size.

I know this is not the ideal solution but it will still allow you to carry a spare in the correct place. Best is to try to modify the existing mount to carry it if you can.
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Old 06-13-2006, 06:16 PM   #8
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If the mismatched tires have a .90 difference one of the tires has to drag that much each revolution to keep up with the other.

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Old 06-20-2006, 05:26 PM   #9
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Hi all, if you do it,it well not last long at all.you would have a bad pull of the steering wheel and hard time keeping it on the road.lower tire pressure will not help on the road as it still has the same circumfrance you can fit it,maybe in the storagr area but noy a good idea for on road. Maybe if you wereto use it till next exit or something like that you might get away with it,But I wouldn't recommend it. Signned:Tire Man retired,also known as Capt. Dan
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Old 06-23-2006, 01:56 PM   #10
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thanks for the replies...I bought the proper size spare 275,70,22.5 tire and plan to mount it underneath between the frame rails with 4 nylon rachet straps...Has anyone already done this and what is the best way to hold the tire with the straps...thanks again...RKL
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