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Old 08-16-2011, 08:01 AM   #1
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Can I mix tire sizes ?

We use 245/75-22.5 tires. No on board mounted spare. I might be able to squeeze a spare in a compartment with a 10 inch high, pass-thru shelf.

Tire dealers tell me it is OK to drive on a 235/80 tire which might fit in the compartment shelf as it is a half-inch less wide.

I'll take the RV to tire stores and dry-fit before I buy. Mounted or un-mounted.

So.....if the smaller tire is installed on the rear, would there be a problem driving?
2003 Dolphin LX 6355 w/ W22 chassis; 8.1L gas & Allison 1000
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Old 08-16-2011, 08:25 AM   #2
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A smaller tire on rear will give an error on you speedometer. A smaller tire will give a reading higher than you are actually going. You'll have a bit more power, but probably not noticeable.
Other than that, no problems.
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Old 08-16-2011, 08:30 AM   #3
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I may have misunderstood. Are you talking about ONE smaller tire in rear? Bad, bad news.
Ok for a spare, but get it off ASAP!
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Old 08-16-2011, 08:36 AM   #4
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You can run a "set" of smaller size tires on a given axle. You just do not want to mix different size tires on the same axle. You also do not want to mix bias and radial tires on the same axle. If you have an odd size spare, you can put it on to use to get to a service center to get the damaged tire fixed/replaced and remounted.

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Old 08-16-2011, 08:43 PM   #5
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Matching two different size tires as dual pair is bad news unless they are very close in effective diameter (and therefore the identical circumference). If one tire in a dual pair is larger than the other, it carries most of the load for that pair and is effectively overloaded. It will self-destruct real soon.

The 235/80 is going to be 4.25 mm more in sidewall height (188mm vs 183.75) which means an extra 8.5mm in diameter. That's about 1/3". That also assumes that both have the same amountof wear. If the 245 had 25k mile son it, its tread would be worn down somewhat, making it that much more smaller than the 235/80 spare, which is probably brand new.

1/3" extra in diameter isn't too bad - I've read that up to 1/2" can be tolerated. However, I wouldn't want to risk driving very far on it.
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Old 08-16-2011, 10:02 PM   #6
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First try this one as we exausted the topic I thinkl.
1/2 inch either way wasnt a problem. Ive RVd since 90 and have done this with no issues


In the search box type tire sizes and you will find lots of discussions about tire diameters mix and match.;
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Old 08-17-2011, 06:14 AM   #7
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Gary is prett much right on. The width of the 245/75 is roughly 9.65". The width of the 235/80 is 9.25". The 235/80 will be .33" (almost 21/64ths) taller than the 245/75.

I don't see any real problem using it as an "emergency spare".
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