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Old 06-26-2004, 12:24 PM   #8
Chuck H
Scooby Specialist
 
Member#: 20442
Join Date: Jun 2002
Chapter/Region: NESIC
Location: Chelmsford, MA
Vehicle:
2022 WRX Limited
WRB

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Quote:
Originally posted by Rally1
To followup, i'll use real numbers from a set of specs on the Falken ST-115's

3.14159265358979323846

245 40 17 = 24.7"
225 45 17 = 25"

77.597" vs 78.540" almost an 1" !


Not good for the diff.
Take that analysis one step further:
245/40-17 = 77.60" circumference = 816.5 revs/mile
225/45-18 = 78.54" circumference = 806.7 revs/mile.

Assuming you're travelling at 60 mph on the highway, the plates in your center differential have a 10 RPM difference in speed. I know that Subaru has that 1/4" overall circumference spec on tire size difference, but that one has always seemed to me to just be a legal trick to always give them an excuse to never replace a diff under warranty. Because if that spec is true, it means that the diff can't tolerate more than a 2.5 RPM rotational difference without being damaged, and in that case, they have no business selling it. If you believe their specs, then an 80 thousandths of an inch difference in tread depth due to uneven tire wear is enough to throw you out of spec and void the warranty on your differential.

I'm not saying that running different sized tires is a good idea, but I don't think it's really as destructive as they'd like you to believe as long as the tires are close.
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