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post #20 of (permalink) Old 01-10-2019, 03:17 PM
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Join Date: Feb 2015
Location: Oklahoma, USA
Car: '15 Outback 2.5i Premium
Posts: 2,134
Originally Posted by Brucey View Post
I was just talking about this to someone else somewhere else.

Subaru supplies their vehicles from the factory with a spare tire that is 3% difference in size.
This handy website says the difference in diameter (and, thus, circumference) is 6%.

Based on the factory sized tire versus the factory sized spare at no point during the tread life of the AT tires should tread depth difference exceed 3%. In fact it's more like half that.

(13/32" brand new on that tire - 2/32" legally end of life = 11/32". 11/32" On a Tire with a Diameter of 29.5" is a difference of around a 1.4% circumference difference.)
Nominal diameter of a 225/65R17 tire is 28.5" according to the tire size website. As someone noted, 11/32" tread wear (1/4" + 3/32") reduces the radius, not diameter, by that amount, from 14 1/4" (14.25") to 13 29/32" (13.90625"), about 2.4%.

I say 3% because that happens to be the size difference between the Subaru factory sized tires (225/65/17) and the factory sized spare (155/80/17) so I am comfortable running it since the newer AWD systems don't require the FWD fuse to be installed and seem to be able to compensate for differences in circumference of at least 3% based on OEM specifications.

Can someone check my math? @Richard Pare @Fibber2

If my math is correct why is it different than the 4/32" quoted within acceptable limits?
See above.

The spare is intended for temporary use at limited speed for a limited distance. Long distances and high speed with mismatched tires is different, even if the mismatch is much less.

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