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Old 12-01-2008, 03:35 PM   #1
Scooby Newbie
Member#: 185310
Join Date: Jul 2008
Chapter/Region: NESIC
Location: Cambridge, MA
2008 Impreza WRX STi

Default Different Tire Diameters Affecting AWD?

I'm shopping around for winter tires, and I had one salesperson swear to me that putting tires/wheels with a different total diameter than what comes stock on the vehicle is bad for the AWD system. He claims that a tire 0.4 inches smaller than OEM is much worse for it than using a tire 0.1 inches larger than OEM. To me, this sounds like a bunch of crap, and I can't think of any legitimate and logical reason that this should be true. Can anyone confirm my doubts or give a valid/logical argument for why he is right?

For those who are mathematically inclined, my car (2008 STi) came with 245/40-18's, which yield:
(width_in_mm * height_width_ratio * 2 sides_of_tire_in_total_diameter) / 25.4 mm_per_inch + inner_dimension_of_tire_in_inches = total diameter of tire in inches
(245mm * 0.4 * 2) / 25.4 + 18in = 25.72in

The 235/45-17's I'm looking at:
(235mm * 0.45 * 2) / 25.4 + 17in = 25.33in
The 225/50-17's this salesperson is trying to sell me:
(225mm * 0.5 * 2) / 25.4 + 17in = 25.86in
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Old 12-01-2008, 03:57 PM   #2
Scooby Specialist
Member#: 45076
Join Date: Oct 2003
Chapter/Region: Tri-State
Location: Rockland County, NY
2004 Stg 2 wagon AT
16 BRZ. Both silver


This is garbage - as long as all of the tires are the same, the AWD system should be fine. Only with different tires or tread wear can you put stress on the AWD diffs and axles, which could lead to failure over time.
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Old 12-01-2008, 04:29 PM   #3
Web Foot STi
Scooby Guru
Member#: 39086
Join Date: Jun 2003
Chapter/Region: NWIC
Location: Portland, OR
2004 STi


Your right. As long as all 4 corners are spinning at the same rpm, going straight down the road, the diffs are happy. Sounds like the salesperson has some tyres to push out the door...
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