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Old 03-20-2008, 09:39 AM   #1
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Tire Pressure Theory

I've been doing some reading on using a higher PSI than the manufacturer recommends. It seems the results are the opposite of what people are experiencing. Instead of wearing out the middle and experiencing "irregular" tread wear, people have reported no unusual wear and even wearing of the tires. Also, blowouts appear to occur for under inflated tires rather than overinflated ones when tire rack and other sites state that overinflated tires were more prone to damage from potholes and other debris.

My theory is that overinflated tires are not experiencing irregular wear due to the driver's hypermiling techniques e.g. driving without brakes and coasting in neutral. If an "aggressive" driver were to drive up to a red light and brake with properly inflated tires and drove in the same manner on overinflated tires, wouldn't he be experiencing irregular wear.
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Old 03-20-2008, 09:57 AM   #2
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I agree with your primary assessments. On blowouts, I have not had any issues from running at 55 psi. On wear, I've seen minor wear on the middle of the tires, but the wear tends to compensate for the wear from turning corners with the front wheel geometry.

My position is that higher inflation seems to be helping a lot more than it's hurting, for me. If someone wants to try it, they are welcome to, and I don't think they will experience a problem.

But, if someone wants me to guarantee they can do something, forget it, they can do their own exeriment, or not gain the benefits.
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Old 03-20-2008, 09:57 AM   #3
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IMO sidewall pressure is maximum recommended cold pressure for tires to survive being driven at 110% of rated speed for 4 hours straight on a 40C day on a harsh road. So you can push it a bit if you intend driving at the speed limit and your temperatures aren't so extreme.

On word of warning though, tires do stretch a bit when you go overpressure on them, it happens gradually though. So you'll put 50 psi in a 35 psi tire and it will be only 45 a couple of days later... then you top it up to 50 again... then maybe it's 47 next time... eventually it will hold 50psi fairly constantly, BUT the tire is now stretched and may not run right at standard pressures again. Also there is potential that you are nearing the elastic limit of the tire and that trying for 60 might cause it to burst catastrophically.

Soooooo... sidewall pressure is the safe cold pressure under all conditions, but messing with that makes you responsible for evaluating conditions.
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Old 03-20-2008, 10:31 AM   #4
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Especially with low-profile tires, a lot of the guys in the Mustang forums complain that they get very fast and uneven tire wear when running the factory pressure, so some of them are even upping their pressures to 40+psi for daily driving, too...
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Old 03-20-2008, 10:37 AM   #5
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Quote:
Originally Posted by KrazyDawg View Post
Instead of wearing out the middle and experiencing "irregular" tread wear, people have reported no unusual wear and even wearing of the tires.
For about the past 1-1/2 years, I work Saturdays at a tire shop. Mostly for fun & stress relief, the money isn't all that.

Anyhow, in the entire time, I have never noticed a set of tires that wore more in the middle than on the edges. Every tire wear pattern favors the edges.

As far as I can tell, the only downside of over-inflation is a harsher ride. I'm sure that "racier" drivers might notice a bit of traction loss also, but I don't roll like that.

-Bob C.
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Old 03-20-2008, 11:33 AM   #6
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A simple formula that seems to work for wear and gas mileage is run the tires at 85% to 90% of the max pressure on the tire sidewall.
If your tire sidewall says max pressure 44 psi as mine do you can safely run 37to 40 psi.
Around town I run 37-38 and when I take long highway trips I up the pressure to 40 psi. I have seen almost no wear problems.
Factory sticker says 36 psi all around.
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Old 03-20-2008, 01:42 PM   #7
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Bob C. You could do an experiment- when a customer comes in with worn out tires, put one in some sort of metal cage (do you all still have those for doing split rim truck tires?) and hook it up to a compressor. Get a safe distance back, turn up the pressure regulator a little at a time and let us know at what pressure it explodes.

That will give us an idea of a tire's no-load "burst" pressure.

But I'm guessing your boss will probably not be too happy with the big BOOM! coming from the shop area.
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Old 03-20-2008, 02:40 PM   #8
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I have almost always run at the max sidewall pressure, if not more. My cars are made for cornering, and one is modified for racing. I take corners pretty fast, usually at cruising speed, and rarely use my brakes compared to most drivers. I've changed 8 tires so far, mostly due to blowouts, and damage from nails, etc... The tires that I've changed because of wear were slightly more worn in the middle, but not significantly. I've seen no loss in traction, but have never seen the skidpad limits of these cars. A couple of times, I had to manuever to not get sideswiped by fools not paying attention, and gripped fine. Braking is fine, too.
I recommend filling your tires at max sidewall, and going to a huge parking lot to test the limits. Maybe put up cones, or something, and testing the car max, and sidewall max.
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Old 03-20-2008, 02:53 PM   #9
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Everything I've driven in the last few years, actually handle better on max sidewall than manufacturer recommended pressure. Done a few emergency lane hops, avoiding ppl trying to pull out into me on 'em and been glad I had them at higher pressure, the quarter second extra response time on a stodgy tire is 22 feet at 60mph, and I've missed the idiots by inches.
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Old 03-20-2008, 03:28 PM   #10
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Even wear at 45 psi.

Here are two pics of Kumho KR21 tires with 2k miles on them. They are on the rear of my Honda. I have run them at 45 psi since day one. The molding bumps are a "superb" wear indicator.
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