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Old 01-21-2014, 06:17 PM   #1 (permalink)
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Increased tire pressure and center wear (tread depth checked after 1 year / 30k mi.)

Ok so it's pretty well established that increased tire pressure increases mileage. We often hear about how awful the handling will be if you increase pressure, and how our tires will be worthless in a few thousand miles as the center wears away quickly.

So I measured my front left tire last winter about this time, drove for a year and accumulated a little under 30,000 miles. The tires were inflated to the max inflation on the sidewall, at 44psi. I maintained this once a month never needing to add more than 4psi to bring it back up. Subaru recommends 26psi. I make no claims here about how much the change has effected mileage as that has been tested elsewhere, and my drive cycle is not anyone else's drive cycle.

Front left tire, cooper CS4 mounted on a Subaru forester.
The tire has 3 grooves running the circumference. one center and one about 1.5 inches from each edge. The measurements were taken at the grooves.

at 169,000 miles on jan 29
Inner
.321"
middle
.322"
outer
.314

average tread depth
.319"

then at 195,000 miles on jan 21
Inner
.245
middle
.240
outer
.224

average tread depth
.236

Change
inner
.076"
middle
.082"
outer
.09"

average wear
.082

I'm not going to make any claims that the uneven treadwear is not going to happen as I don't have a large data set but on this 1 data point it does not seem to be an issue. What these measurements do tell me is that I need to rotate my tires and do an alignment.

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Last edited by bestclimb; 01-21-2014 at 09:09 PM.. Reason: fix typo
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Old 01-21-2014, 07:06 PM   #2 (permalink)
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Thanks for posting your data!

My experience (Pontiac Firefly/Chev Metro):

- 155/80r13 tires
- 30,000 km / ~20,000 miles
- PSI significantly above sidewall maximum (44 PSI)

No obvious center wear (by eyeball, not by depth gauge).

I won't be surprised to learn there are some vehicle/tire combinations where the tires have worn in the center from high pressure. Best advice is to monitor/measure rather than set & forget.
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Old 01-21-2014, 07:16 PM   #3 (permalink)
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Whatever pushes out on the center also pushes out equally on both sides. Typically front tires wear out on the outer edges from cornering. Rotate front to rear on same side.
In 100k + miles (several vehicles and tire types) on tires inflated to 44 PSI I have seen no sign of center wear, in fact the tires last a good bit longer at those pressures.

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Old 01-21-2014, 08:00 PM   #4 (permalink)
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I can never get the center to wear out faster than the outside edge. This is probably because I like to drive without brakes, which means I'm carrying a lot of corner speed.

I was hoping that over-inflation would show increased center wear so that I can use that strategy to even out my tire wear.
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Old 01-21-2014, 08:02 PM   #5 (permalink)
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I agree.

I have always had my tires between 35-38lbs and in the last year and a half went to 40-44lbs. No abnormal wear and the tires last longer, no rounded shoulders.
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Old 01-22-2014, 02:29 AM   #6 (permalink)
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I had 175/70-13 Blizzaks wear in the center at 35 PSI (sidewall max). I'm now running 155/80-13 Blizzaks (same model) at 40 PSI. Wear seems fairy even, maybe because they're skinnier?

My 185/65-14 summer tires have more wear on the sides at 55-60 PSI. Sidewall max is 44 PSI. They're used tires, so some of the uneven wear could've come with them.
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Old 01-22-2014, 06:59 AM   #7 (permalink)
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I should measure mine. I do not have original measurements, but brand new tires, I imagine they were even.

One thing I thought was when the center wears down a bit, there has to be a terminal point to when the sides start wearing down, too.

Is anyone else seeing their rear tires wear down faster than their front tires?
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Old 01-22-2014, 08:54 AM   #8 (permalink)
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Subaru?--Might want to check that front wheel alignment

Quote:
Originally Posted by bestclimb View Post
Front left tire, cooper CS4 mounted on a Subaru forester.
at 169,000 miles on jan 29
average tread depth
.319"

then at 195,000 miles on jan 21
average tread depth
.236

Change
inner
.076"
middle
.082"
outer
.09"

average wear
.082

... What these measurements do tell me is that I need to rotate my tires and do an alignment.
That's some good data, thanks for sharing.

i noticed years ago that Subaru ran positive camber on the front wheels--never understood why since it grinds up the front outer edges. The late models such as yours probably don't do that anymore, but doesn't hurt to check.

Over 26k miles you are wearing at .003 inch per 1k miles so you will get about 58k miles down to 1/16. The outer-to-inner wear wedge of .014 looks like excessive toe-in to me, camber wear is much worse. But even zero toe with a slight positive camber could give you that also. So measure the alignment and post up the results.
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Old 01-22-2014, 03:42 PM   #9 (permalink)
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Quote:
Originally Posted by bestclimb View Post
We often hear about how awful the handling will be if you increase pressure...
Only by people who have never tried it, or have only tried it on competition-grade tires or racing slicks. Every street tire I have tried it on (so far about a dozen brands/models/sizes) has given me better handling at higher pressures than the placard pressures. The transient response, in particular, was hugely improved--but the ultimate grip level was better as well.



Quote:
Originally Posted by kennybobby View Post
i noticed years ago that Subaru ran positive camber on the front wheels--never understood why since it grinds up the front outer edges.
Probably to promote understeer. If the nose of the car hits the wall, it's your fault for driving too fast. If the tail hits the wall, it's the car maker's fault for selling you an unstable car... At least, that seems to be the opinion of several courts over the years. Understeer is viewed as safe and easy to control, while oversteer is viewed as unsafe and difficult to control.

I prefer to be able to get either one depending on how I am driving, but I'm a little odd in my preferences.

-soD
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Old 02-26-2016, 04:55 PM   #10 (permalink)
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I was going to wait to post this because I wanted to look in my records to see how many years and miles I have on this set of tires but... heck with it.

I stopped by a tire shop to borrow their tread depth gauge a while back to see how my inflation strategy was working. F150, tires only a few years old, I'd guess less than 20,000 miles. From new I tried 50 psi (44 sidewall max) but it rode like it had solid tires so I backed off to 46, which they've been at practically the whole time. The center is wearing faster than either of the sides, front and rear, on both sides. So I'm going to back off at least 4 psi and see how that goes.

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